diff --git a/Project.toml b/Project.toml
index ed7a99f8d..92e5f0e4a 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "SymbolicUtils"
 uuid = "d1185830-fcd6-423d-90d6-eec64667417b"
 authors = ["Shashi Gowda"]
-version = "1.5.1"
+version = "1.6.0"
 
 [deps]
 AbstractTrees = "1520ce14-60c1-5f80-bbc7-55ef81b5835c"
@@ -22,6 +22,7 @@ SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
 StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"
 SymbolicIndexingInterface = "2efcf032-c050-4f8e-a9bb-153293bab1f5"
+TermInterface = "8ea1fca8-c5ef-4a55-8b96-4e9afe9c9a3c"
 TimerOutputs = "a759f4b9-e2f1-59dc-863e-4aeb61b1ea8f"
 Unityper = "a7c27f48-0311-42f6-a7f8-2c11e75eb415"
 
@@ -42,6 +43,7 @@ Setfield = "0.7, 0.8, 1"
 SpecialFunctions = "0.10, 1.0, 2"
 StaticArrays = "0.12, 1.0"
 SymbolicIndexingInterface = "0.3"
+TermInterface = "0.4"
 TimerOutputs = "0.5"
 Unityper = "0.1.2"
 julia = "1.3"
diff --git a/docs/src/api.md b/docs/src/api.md
index eb93f3ee0..45266df82 100644
--- a/docs/src/api.md
+++ b/docs/src/api.md
@@ -12,15 +12,6 @@ SymbolicUtils.Pow
 SymbolicUtils.promote_symtype
 ```
 
-## Interfacing
-
-```@docs
-SymbolicUtils.istree
-SymbolicUtils.operation
-SymbolicUtils.arguments
-SymbolicUtils.similarterm
-```
-
 ## Rewriters
 
 ```@docs
diff --git a/docs/src/index.md b/docs/src/index.md
index f7cc0a7f7..50829f591 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -108,15 +108,8 @@ g(2//5, g(1, β))
 
 Symbolic expressions are of type `Term{T}`, `Add{T}`, `Mul{T}`, `Pow{T}` or `Div{T}` and denote some function call where one or more arguments are themselves such expressions or `Sym`s. See more about the representation [here](/representation/).
 
-All the expression types support the following:
-
-- `istree(x)` -- always returns `true` denoting, `x` is not a leaf node like Sym or a literal.
-- `operation(x)` -- the function being called
-- `arguments(x)` -- a vector of arguments
-- `symtype(x)` -- the "inferred" type (`T`)
-
-See more on the interface [here](/interface)
-
+All the expression types support the [TermInterface.jl](https://github.com/0x0f0f0f/TermInterface.jl) interface.
+Please refer to the package for the complete reference of the interface.
 
 ## Term rewriting
 
diff --git a/src/SymbolicUtils.jl b/src/SymbolicUtils.jl
index d7e6a6a80..286ba3bc1 100644
--- a/src/SymbolicUtils.jl
+++ b/src/SymbolicUtils.jl
@@ -4,21 +4,26 @@ $(DocStringExtensions.README)
 module SymbolicUtils
 
 using DocStringExtensions
+
 export @syms, term, showraw, hasmetadata, getmetadata, setmetadata
 
 using Unityper
-
-# Sym, Term,
-# Add, Mul and Pow
+using TermInterface
 using DataStructures
 using Setfield
 import Setfield: PropertyLens
 using SymbolicIndexingInterface
 import Base: +, -, *, /, //, \, ^, ImmutableDict
 using ConstructionBase
-include("interface.jl")
+using TermInterface
+import TermInterface: iscall, isexpr, issym, symtype, head, children,
+                      operation, arguments, metadata, maketerm
+
+Base.@deprecate_binding istree iscall
+export istree, operation, arguments, unsorted_arguments, similarterm
+# Sym, Term,
+# Add, Mul and Pow
 include("types.jl")
-export istree, operation, arguments, similarterm
 
 # Methods on symbolic objects
 using SpecialFunctions, NaNMath
diff --git a/src/code.jl b/src/code.jl
index 7c647ba9c..c262fc8ed 100644
--- a/src/code.jl
+++ b/src/code.jl
@@ -8,7 +8,7 @@ export toexpr, Assignment, (←), Let, Func, DestructuredArgs, LiteralExpr,
 
 import ..SymbolicUtils
 import ..SymbolicUtils.Rewriters
-import SymbolicUtils: @matchable, BasicSymbolic, Sym, Term, istree, operation, arguments, issym,
+import SymbolicUtils: @matchable, BasicSymbolic, Sym, Term, iscall, operation, arguments, issym,
                       symtype, similarterm, unsorted_arguments, metadata, isterm, term
 
 ##== state management ==##
@@ -162,7 +162,7 @@ end
 toexpr(O::Expr, st) = O
 
 function substitute_name(O, st)
-    if (issym(O) || istree(O)) && haskey(st.rewrites, O)
+    if (issym(O) || iscall(O)) && haskey(st.rewrites, O)
         st.rewrites[O]
     else
         O
@@ -176,13 +176,13 @@ function toexpr(O, st)
     end
     O = substitute_name(O, st)
 
-    !istree(O) && return O
+    !iscall(O) && return O
     op = operation(O)
     expr′ = function_to_expr(op, O, st)
     if expr′ !== nothing
         return expr′
     else
-        !istree(O) && return O
+        !iscall(O) && return O
         args = arguments(O)
         return Expr(:call, toexpr(op, st), map(x->toexpr(x, st), args)...)
     end
@@ -221,7 +221,7 @@ get_rewrites(args::DestructuredArgs) = ()
 function get_rewrites(args::Union{AbstractArray, Tuple})
     cflatten(map(get_rewrites, args))
 end
-get_rewrites(x) = istree(x) ? (x,) : ()
+get_rewrites(x) = iscall(x) ? (x,) : ()
 cflatten(x) = Iterators.flatten(x) |> collect
 
 # Used in Symbolics
@@ -691,7 +691,7 @@ end
 @inline newsym(::Type{T}) where T = Sym{T}(gensym("cse"))
 
 function _cse!(mem, expr)
-    istree(expr) || return expr
+    iscall(expr) || return expr
     op = _cse!(mem, operation(expr))
     args = map(Base.Fix1(_cse!, mem), arguments(expr))
     t = similarterm(expr, op, args)
@@ -742,7 +742,7 @@ end
 
 
 function cse_state!(state, t)
-    !istree(t) && return t
+    !iscall(t) && return t
     state[t] = Base.get!(state, t, 0) + 1
     foreach(x->cse_state!(state, x), unsorted_arguments(t))
 end
@@ -758,7 +758,7 @@ function cse_block!(assignments, counter, names, name, state, x)
             counter[] += 1
             return sym
         end
-    elseif istree(x)
+    elseif iscall(x)
         args = map(a->cse_block!(assignments, counter, names, name, state,a), unsorted_arguments(x))
         if isterm(x)
             return term(operation(x), args...)
diff --git a/src/inspect.jl b/src/inspect.jl
index f62551893..42b0b1be5 100644
--- a/src/inspect.jl
+++ b/src/inspect.jl
@@ -2,11 +2,11 @@ import AbstractTrees
 
 const inspect_metadata = Ref{Bool}(false)
 function AbstractTrees.nodevalue(x::Symbolic)
-    istree(x) ? operation(x) : x
+    iscall(x) ? operation(x) : isexpr(x) ? head(x) : x
 end
 
 function AbstractTrees.nodevalue(x::BasicSymbolic)
-    str = if !istree(x)
+    str = if !iscall(x)
         string(exprtype(x), "(", x, ")")
     elseif isadd(x)
         string(exprtype(x), 
@@ -27,7 +27,7 @@ function AbstractTrees.nodevalue(x::BasicSymbolic)
 end
 
 function AbstractTrees.children(x::Symbolic)
-    istree(x) ? arguments(x) : ()
+    iscall(x) ? arguments(x) : isexpr(x) ? children(x) : ()
 end
 
 """
diff --git a/src/interface.jl b/src/interface.jl
index 255ef584f..687a802a8 100644
--- a/src/interface.jl
+++ b/src/interface.jl
@@ -1,10 +1,10 @@
 """
-  istree(x)
+  iscall(x)
 
 Returns `true` if `x` is a term. If true, `operation`, `arguments`
 must also be defined for `x` appropriately.
 """
-istree(x) = false
+iscall(x) = false
 
 """
   symtype(x)
@@ -29,7 +29,7 @@ issym(x) = false
 """
   operation(x)
 
-If `x` is a term as defined by `istree(x)`, `operation(x)` returns the
+If `x` is a term as defined by `iscall(x)`, `operation(x)` returns the
 head of the term if `x` represents a function call, for example, the head
 is the function being called.
 """
@@ -38,14 +38,14 @@ function operation end
 """
   arguments(x)
 
-Get the arguments of `x`, must be defined if `istree(x)` is `true`.
+Get the arguments of `x`, must be defined if `iscall(x)` is `true`.
 """
 function arguments end
 
 """
   unsorted_arguments(x::T)
 
-If x is a term satisfying `istree(x)` and your term type `T` provides
+If x is a term satisfying `iscall(x)` and your term type `T` provides
 an optimized implementation for storing the arguments, this function can
 be used to retrieve the arguments when the order of arguments does not matter 
 but the speed of the operation does.
diff --git a/src/matchers.jl b/src/matchers.jl
index 531bc1535..7f4dea537 100644
--- a/src/matchers.jl
+++ b/src/matchers.jl
@@ -6,7 +6,7 @@
 # 3. Callback: takes arguments Dictionary × Number of elements matched
 #
 function matcher(val::Any)
-    istree(val) && return term_matcher(val)
+    iscall(val) && return term_matcher(val)
     function literal_matcher(next, data, bindings)
         islist(data) && isequal(car(data), val) ? next(bindings, 1) : nothing
     end
@@ -89,7 +89,7 @@ function term_matcher(term)
     function term_matcher(success, data, bindings)
 
         !islist(data) && return nothing
-        !istree(car(data)) && return nothing
+        !iscall(car(data)) && return nothing
 
         function loop(term, bindings′, matchers′) # Get it to compile faster
             if !islist(matchers′)
diff --git a/src/ordering.jl b/src/ordering.jl
index 81d64a9b4..3417f3f85 100644
--- a/src/ordering.jl
+++ b/src/ordering.jl
@@ -20,7 +20,7 @@
 function get_degrees(expr)
     if issym(expr)
         ((Symbol(expr),) => 1,)
-    elseif istree(expr)
+    elseif iscall(expr)
         op = operation(expr)
         args = arguments(expr)
         if operation(expr) == (^) && args[2] isa Number
@@ -62,7 +62,7 @@ function lexlt(degs1, degs2)
     return false # they are equal
 end
 
-_arglen(a) = istree(a) ? length(unsorted_arguments(a)) : 0
+_arglen(a) = iscall(a) ? length(unsorted_arguments(a)) : 0
 
 function <ₑ(a::Tuple, b::Tuple)
     for (x, y) in zip(a, b)
diff --git a/src/polyform.jl b/src/polyform.jl
index 873e332a8..21e04ac9b 100644
--- a/src/polyform.jl
+++ b/src/polyform.jl
@@ -6,7 +6,7 @@ using Bijections
 
 Abstracts a [MultivariatePolynomials.jl](https://juliaalgebra.github.io/MultivariatePolynomials.jl/stable/) as a SymbolicUtils expression and vice-versa.
 
-The SymbolicUtils term interface (`istree`, `operation, and `arguments`) works on PolyForm lazily:
+The SymbolicUtils term interface (`isexpr`/`iscall`, `operation, and `arguments`) works on PolyForm lazily:
 the `operation` and `arguments` are created by converting one level of arguments into SymbolicUtils expressions. They may further contain PolyForm within them.
 We use this to hold polynomials in memory while doing `simplify_fractions`.
 
@@ -97,7 +97,7 @@ _isone(p::PolyForm) = isone(p.p)
 function polyize(x, pvar2sym, sym2term, vtype, pow, Fs, recurse)
     if x isa Number
         return x
-    elseif istree(x)
+    elseif iscall(x)
         if !(symtype(x) <: Number)
             error("Cannot convert $x of symtype $(symtype(x)) into a PolyForm")
         end
@@ -170,8 +170,10 @@ function PolyForm(x,
     PolyForm{symtype(x)}(p, pvar2sym, sym2term, metadata)
 end
 
-istree(x::Type{<:PolyForm}) = true
-istree(x::PolyForm) = true
+isexpr(x::Type{<:PolyForm}) = true
+isexpr(x::PolyForm) = true
+iscall(x::Type{<:PolyForm}) = true
+iscall(x::PolyForm) = true
 
 function similarterm(t::PolyForm, f, args, symtype; metadata=nothing)
     basic_similarterm(t, f, args, symtype; metadata=metadata)
@@ -181,6 +183,7 @@ function similarterm(::PolyForm, f::Union{typeof(*), typeof(+), typeof(^)},
     f(args...)
 end
 
+head(::PolyForm) = PolyForm
 operation(x::PolyForm) = MP.nterms(x.p) == 1 ? (*) : (+)
 
 function arguments(x::PolyForm{T}) where {T}
@@ -227,6 +230,7 @@ function arguments(x::PolyForm{T}) where {T}
                  PolyForm{T}(t, x.pvar2sym, x.sym2term, nothing)) for t in ts]
     end
 end
+children(x::PolyForm) = [operation(x); arguments(x)]
 
 Base.show(io::IO, x::PolyForm) = show_term(io, x)
 
@@ -336,7 +340,7 @@ function simplify_fractions(x; polyform=false)
 end
 
 function add_with_div(x, flatten=true)
-    (!istree(x) || operation(x) != (+)) && return x
+    (!iscall(x) || operation(x) != (+)) && return x
     aa = unsorted_arguments(x)
     !any(a->isdiv(a), aa) && return x # no rewrite necessary
 
@@ -361,7 +365,7 @@ function flatten_fractions(x)
 end
 
 function fraction_iszero(x)
-    !istree(x) && return _iszero(x)
+    !iscall(x) && return _iszero(x)
     ff = flatten_fractions(x)
     # fast path and then slow path
     any(_iszero, numerators(ff)) ||
@@ -369,18 +373,18 @@ function fraction_iszero(x)
 end
 
 function fraction_isone(x)
-    !istree(x) && return _isone(x)
+    !iscall(x) && return _isone(x)
     _isone(simplify_fractions(flatten_fractions(x)))
 end
 
 function needs_div_rules(x)
     (isdiv(x) && !(x.num isa Number) && !(x.den isa Number)) ||
-    (istree(x) && operation(x) === (+) && count(has_div, unsorted_arguments(x)) > 1) ||
-    (istree(x) && any(needs_div_rules, unsorted_arguments(x)))
+    (iscall(x) && operation(x) === (+) && count(has_div, unsorted_arguments(x)) > 1) ||
+    (iscall(x) && any(needs_div_rules, unsorted_arguments(x)))
 end
 
 function has_div(x)
-    return isdiv(x) || (istree(x) && any(has_div, unsorted_arguments(x)))
+    return isdiv(x) || (iscall(x) && any(has_div, unsorted_arguments(x)))
 end
 
 flatten_pows(xs) = map(xs) do x
diff --git a/src/rewriters.jl b/src/rewriters.jl
index fffd3ee95..464898d36 100644
--- a/src/rewriters.jl
+++ b/src/rewriters.jl
@@ -31,8 +31,9 @@ rewriters.
 """
 module Rewriters
 using SymbolicUtils: @timer
+using TermInterface
 
-import SymbolicUtils: similarterm, istree, operation, arguments, unsorted_arguments, node_count
+import SymbolicUtils: similarterm
 export Empty, IfElse, If, Chain, RestartedChain, Fixpoint, Postwalk, Prewalk, PassThrough
 
 # Cache of printed rules to speed up @timer
@@ -190,11 +191,11 @@ instrument(x::PassThrough, f) = PassThrough(instrument(x.rw, f))
 passthrough(x, default) = x === nothing ? default : x
 function (p::Walk{ord, C, F, false})(x) where {ord, C, F}
     @assert ord === :pre || ord === :post
-    if istree(x)
+    if iscall(x)
         if ord === :pre
             x = p.rw(x)
         end
-        if istree(x)
+        if iscall(x)
             x = p.similarterm(x, operation(x), map(PassThrough(p), unsorted_arguments(x)))
         end
         return ord === :post ? p.rw(x) : x
@@ -205,11 +206,11 @@ end
 
 function (p::Walk{ord, C, F, true})(x) where {ord, C, F}
     @assert ord === :pre || ord === :post
-    if istree(x)
+    if iscall(x)
         if ord === :pre
             x = p.rw(x)
         end
-        if istree(x)
+        if iscall(x)
             _args = map(arguments(x)) do arg
                 if node_count(arg) > p.thread_cutoff
                     Threads.@spawn p(arg)
diff --git a/src/rule.jl b/src/rule.jl
index a872222e6..a529979e7 100644
--- a/src/rule.jl
+++ b/src/rule.jl
@@ -120,7 +120,7 @@ end
 getdepth(r::Rule) = r.depth
 
 function rule_depth(rule, d=0, maxdepth=0)
-    if istree(rule)
+    if iscall(rule)
         maxdepth = reduce(max, (rule_depth(r, d+1, maxdepth) for r in arguments(rule)), init=1)
     elseif rule isa Slot || rule isa Segment
         maxdepth = max(d, maxdepth)
@@ -389,7 +389,7 @@ Base.show(io::IO, acr::ACRule) = print(io, "ACRule(", acr.rule, ")")
 
 function (acr::ACRule)(term)
     r = Rule(acr)
-    if !istree(term)
+    if !iscall(term)
         r(term)
     else
         f =  operation(term)
diff --git a/src/simplify.jl b/src/simplify.jl
index 0c0bfd44a..87bc95954 100644
--- a/src/simplify.jl
+++ b/src/simplify.jl
@@ -43,7 +43,7 @@ function simplify(x;
         SymbolicUtils.simplify_fractions(x) : x
 end
 
-has_operation(x, op) = (istree(x) && (operation(x) == op ||
+has_operation(x, op) = (iscall(x) && (operation(x) == op ||
                                       any(a->has_operation(a, op),
                                           unsorted_arguments(x))))
 
diff --git a/src/simplify_rules.jl b/src/simplify_rules.jl
index b12a9f272..a612036cb 100644
--- a/src/simplify_rules.jl
+++ b/src/simplify_rules.jl
@@ -2,9 +2,9 @@ using .Rewriters
 """
   is_operation(f)
 Returns a single argument anonymous function predicate, that returns `true` if and only if
-the argument to the predicate satisfies `istree` and `operation(x) == f` 
+the argument to the predicate satisfies `iscall` and `operation(x) == f` 
 """
-is_operation(f) = @nospecialize(x) -> istree(x) && (operation(x) == f)
+is_operation(f) = @nospecialize(x) -> iscall(x) && (operation(x) == f)
 
 let
     CANONICALIZE_PLUS = [
@@ -132,7 +132,7 @@ let
     ]
 
     function number_simplifier()
-        rule_tree = [If(istree, Chain(ASSORTED_RULES)),
+        rule_tree = [If(iscall, Chain(ASSORTED_RULES)),
                      If(x -> !isadd(x) && is_operation(+)(x),
                         Chain(CANONICALIZE_PLUS)),
                      If(is_operation(+), Chain(PLUS_DISTRIBUTE)), # This would be useful even if isadd
@@ -173,12 +173,12 @@ let
     end
 
     # reduce overhead of simplify by defining these as constant
-    serial_simplifier = If(istree, Fixpoint(default_simplifier()))
+    serial_simplifier = If(iscall, Fixpoint(default_simplifier()))
 
     threaded_simplifier(cutoff) = Fixpoint(default_simplifier(threaded=true,
                                                               thread_cutoff=cutoff))
 
-    serial_expand_simplifier = If(istree,
+    serial_expand_simplifier = If(iscall,
                                   Fixpoint(Chain((expand,
                                                   Fixpoint(default_simplifier())))))
 
diff --git a/src/substitute.jl b/src/substitute.jl
index 9a5213f0b..73ea7659d 100644
--- a/src/substitute.jl
+++ b/src/substitute.jl
@@ -16,7 +16,7 @@ julia> substitute(1+sqrt(y), Dict(y => 2), fold=false)
 function substitute(expr, dict; fold=true)
     haskey(dict, expr) && return dict[expr]
 
-    if istree(expr)
+    if iscall(expr)
         op = substitute(operation(expr), dict; fold=fold)
         if fold
             canfold = !(op isa Symbolic)
@@ -53,7 +53,7 @@ Base.occursin(needle::Symbolic, haystack) = _occursin(needle, haystack)
 function _occursin(needle, haystack)
     isequal(needle, haystack) && return true
 
-    if istree(haystack)
+    if iscall(haystack)
         args = arguments(haystack)
         for arg in args
             occursin(needle, arg) && return true
diff --git a/src/types.jl b/src/types.jl
index 2202a8df1..f9ea33121 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -114,6 +114,8 @@ symtype(x::Number) = typeof(x)
     end
 end
 
+@inline head(x::BasicSymbolic) = BasicSymbolic
+
 function arguments(x::BasicSymbolic)
     args = unsorted_arguments(x)
     @compactified x::BasicSymbolic begin
@@ -135,6 +137,9 @@ function arguments(x::BasicSymbolic)
     end
     return args
 end
+
+unsorted_arguments(x) = arguments(x)
+children(x::BasicSymbolic) = [operation(x); arguments(x)]
 function unsorted_arguments(x::BasicSymbolic)
     @compactified x::BasicSymbolic begin
         Term => return x.arguments
@@ -157,7 +162,7 @@ function unsorted_arguments(x::BasicSymbolic)
     if isadd(x)
         for (k, v) in x.dict
             push!(args, applicable(*,k,v) ? k*v :
-                    similarterm(k, *, [k, v]))
+                    maketerm(k, *, [k, v]))
         end
     else # MUL
         for (k, v) in x.dict
@@ -183,7 +188,9 @@ function unsorted_arguments(x::BasicSymbolic)
     return args
 end
 
-istree(s::BasicSymbolic) = !issym(s)
+isexpr(s::BasicSymbolic) = !issym(s)
+iscall(s::BasicSymbolic) = isexpr(s)
+
 @inline isa_SymType(T::Val{S}, x) where {S} = x isa BasicSymbolic ? Unityper.isa_type_fun(Val(SymbolicUtils.BasicSymbolic), T, x) : false
 issym(x::BasicSymbolic) = isa_SymType(Val(:Sym), x)
 isterm(x) = isa_SymType(Val(:Term), x)
@@ -400,7 +407,7 @@ end
 
 @inline function numerators(x)
     isdiv(x) && return numerators(x.num)
-    istree(x) && operation(x) === (*) ? arguments(x) : Any[x]
+    iscall(x) && operation(x) === (*) ? arguments(x) : Any[x]
 end
 
 @inline denominators(x) = isdiv(x) ? numerators(x.den) : Any[1]
@@ -516,7 +523,7 @@ end
 Binarizes `Term`s with n-ary operations
 """
 function unflatten(t::Symbolic{T}) where{T}
-    if istree(t)
+    if iscall(t)
         f = operation(t)
         if f == (+) || f == (*)   # TODO check out for other n-ary --> binary ops
             a = arguments(t)
@@ -528,28 +535,12 @@ end
 
 unflatten(t) = t
 
-"""
-    similarterm(t, f, args, symtype; metadata=nothing)
-
-Create a term that is similar in type to `t`. Extending this function allows packages
-using their own expression types with SymbolicUtils to define how new terms should
-be created. Note that `similarterm` may return an object that has a
-different type than `t`, because `f` also influences the result.
-
-## Arguments
+function TermInterface.maketerm(::Type{<:BasicSymbolic}, head, args, type, metadata)
+    basicsymbolic(first(args), args[2:end], type, metadata)
+end
 
-- `t` the reference term to use to create similar terms
-- `f` is the operation of the term
-- `args` is the arguments
-- The `symtype` of the resulting term. Best effort will be made to set the symtype of the
-  resulting similar term to this type.
-"""
-similarterm(t::Symbolic, f, args; metadata=nothing) =
-    similarterm(t, f, args, _promote_symtype(f, args); metadata=metadata)
-similarterm(t::BasicSymbolic, f, args,
-            symtype; metadata=nothing) = basic_similarterm(t, f, args, symtype; metadata=metadata)
 
-function basic_similarterm(t, f, args, stype; metadata=nothing)
+function basicsymbolic(f, args, stype, metadata)
     if f isa Symbol
         error("$f must not be a Symbol")
     end
@@ -559,7 +550,7 @@ function basic_similarterm(t, f, args, stype; metadata=nothing)
     end
     if T <: LiteralReal
         Term{T}(f, args, metadata=metadata)
-    elseif stype <: Number && (f in (+, *) || (f in (/, ^) && length(args) == 2)) && all(x->symtype(x) <: Number, args)
+    elseif T <: Number && (f in (+, *) || (f in (/, ^) && length(args) == 2)) && all(x->symtype(x) <: Number, args)
         res = f(args...)
         if res isa Symbolic
             @set! res.metadata = metadata
@@ -580,16 +571,17 @@ function hasmetadata(s::Symbolic, ctx)
     metadata(s) isa AbstractDict && haskey(metadata(s), ctx)
 end
 
-function issafecanon(f, s)
+issafecanon(f, s) = true
+function issafecanon(f, s::Symbolic)
     if isnothing(metadata(s)) || issym(s)
         return true
     else
         _issafecanon(f, s)
     end
 end
-_issafecanon(::typeof(*), s) = !istree(s) || !(operation(s) in (+,*,^))
-_issafecanon(::typeof(+), s) = !istree(s) || !(operation(s) in (+,*))
-_issafecanon(::typeof(^), s) = !istree(s) || !(operation(s) in (*, ^))
+_issafecanon(::typeof(*), s) = !iscall(s) || !(operation(s) in (+,*,^))
+_issafecanon(::typeof(+), s) = !iscall(s) || !(operation(s) in (+,*))
+_issafecanon(::typeof(^), s) = !iscall(s) || !(operation(s) in (*, ^))
 
 issafecanon(f, ss...) = all(x->issafecanon(f, x), ss)
 
@@ -641,12 +633,42 @@ end
 
 function to_symbolic(x)
     Base.depwarn("`to_symbolic(x)` is deprecated, define the interface for your " *
-                 "symbolic structure using `istree(x)`, `operation(x)`, `arguments(x)` " *
+                 "symbolic structure using `iscall(x)`, `operation(x)`, `arguments(x)` " *
                  "and `similarterm(::YourType, f, args, symtype)`", :to_symbolic, force=true)
 
     x
 end
 
+"""
+    similarterm(x, op, args, symtype=nothing; metadata=nothing)
+
+"""
+function similarterm(x, op, args, symtype=nothing; metadata=nothing)
+  Base.depwarn("""`similarterm` is deprecated, use `maketerm` instead.
+                  See https://github.com/JuliaSymbolics/TermInterface.jl for details.
+                  The present call can be replaced by
+                  `maketerm(typeof(x), $(head(x)), [op, args...], symtype, metadata)`""", :similarterm)
+
+  TermInterface.maketerm(typeof(x), callhead(x), [op, args...], symtype, metadata)
+end
+
+# Old fallback
+function similarterm(T::Type, op, args, symtype=nothing; metadata=nothing)
+  Base.depwarn("`similarterm` is deprecated, use `maketerm` instead." *
+               "See https://github.com/JuliaSymbolics/TermInterface.jl for details.", :similarterm)
+  op(args...)
+end
+
+export similarterm
+
+
+"""
+    callhead(x)
+Used in this deprecation cycle of `similarterm` to find the `head` argument to
+`maketerm`. Do not implement this, or use `similarterm` if you're using this package.
+"""
+callhead(x) = typeof(x)
+
 ###
 ###  Pretty printing
 ###
@@ -654,7 +676,7 @@ const show_simplified = Ref(false)
 
 isnegative(t::Real) = t < 0
 function isnegative(t)
-    if istree(t) && operation(t) === (*)
+    if iscall(t) && operation(t) === (*)
         coeff = first(arguments(t))
         return isnegative(coeff)
     end
@@ -666,7 +688,7 @@ setargs(t, args) = Term{symtype(t)}(operation(t), args)
 cdrargs(args) = setargs(t, cdr(args))
 
 print_arg(io, x::Union{Complex, Rational}; paren=true) = print(io, "(", x, ")")
-isbinop(f) = istree(f) && !istree(operation(f)) && Base.isbinaryoperator(nameof(operation(f)))
+isbinop(f) = iscall(f) && !iscall(operation(f)) && Base.isbinaryoperator(nameof(operation(f)))
 function print_arg(io, x; paren=false)
     if paren && isbinop(x)
         print(io, "(", x, ")")
@@ -685,7 +707,7 @@ function print_arg(io, f, x)
 end
 
 function remove_minus(t)
-    !istree(t) && return -t
+    !iscall(t) && return -t
     @assert operation(t) == (*)
     args = arguments(t)
     @assert args[1] < 0
@@ -756,9 +778,9 @@ function show_ref(io, f, args)
     x = args[1]
     idx = args[2:end]
 
-    istree(x) && print(io, "(")
+    iscall(x) && print(io, "(")
     print(io, x)
-    istree(x) && print(io, ")")
+    iscall(x) && print(io, ")")
     print(io, "[")
     for i=1:length(idx)
         print_arg(io, idx[i])
@@ -768,7 +790,7 @@ function show_ref(io, f, args)
 end
 
 function show_call(io, f, args)
-    fname = istree(f) ? Symbol(repr(f)) : nameof(f)
+    fname = iscall(f) ? Symbol(repr(f)) : nameof(f)
     len_args = length(args)
     if Base.isunaryoperator(fname) && len_args == 1
         print(io, "$fname")
@@ -810,7 +832,7 @@ function show_term(io::IO, t)
         show_pow(io, args)
     elseif f === (getindex)
         show_ref(io, f, args)
-    elseif f === (identity) && !issym(args[1]) && !istree(args[1])
+    elseif f === (identity) && !issym(args[1]) && !iscall(args[1])
         show(io, args[1])
     else
         show_call(io, f, args)
diff --git a/src/utils.jl b/src/utils.jl
index acf9e92d6..90d7c407c 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -35,7 +35,7 @@ pow(x::Symbolic,y::Symbolic) = Base.:^(x,y)
 
 # Simplification utilities
 function has_trig_exp(term)
-    !istree(term) && return false
+    !iscall(term) && return false
     fns = (sin, cos, tan, cot, sec, csc, exp, cosh, sinh)
     op = operation(term)
 
@@ -47,7 +47,7 @@ function has_trig_exp(term)
 end
 
 function fold(t)
-    if istree(t)
+    if iscall(t)
         tt = map(fold, arguments(t))
         if !any(x->x isa Symbolic, tt)
             # evaluate it
@@ -81,7 +81,7 @@ function isnotflat(⋆)
     function (x)
         args = arguments(x)
         for t in args
-            if istree(t) && operation(t) === (⋆)
+            if iscall(t) && operation(t) === (⋆)
                 return true
             end
         end
@@ -141,7 +141,7 @@ function flatten_term(⋆, x)
     # flatten nested ⋆
     flattened_args = []
     for t in args
-        if istree(t) && operation(t) === (⋆)
+        if iscall(t) && operation(t) === (⋆)
             append!(flattened_args, arguments(t))
         else
             push!(flattened_args, t)
@@ -170,7 +170,7 @@ struct LL{V}
     i::Int
 end
 
-islist(x) = istree(x) || !isempty(x)
+islist(x) = iscall(x) || !isempty(x)
 
 Base.empty(l::LL) = empty(l.v)
 Base.isempty(l::LL) = l.i > length(l.v)
@@ -184,9 +184,9 @@ Base.isempty(t::Term) = false
 @inline car(t::Term) = operation(t)
 @inline cdr(t::Term) = arguments(t)
 
-@inline car(v) = istree(v) ? operation(v) : first(v)
+@inline car(v) = iscall(v) ? operation(v) : first(v)
 @inline function cdr(v)
-    if istree(v)
+    if iscall(v)
         arguments(v)
     else
         islist(v) ? LL(v, 2) : error("asked cdr of empty")
@@ -200,7 +200,7 @@ end
     if n === 0
         return ll
     else
-        istree(ll) ? drop_n(arguments(ll), n-1) : drop_n(cdr(ll), n-1)
+        iscall(ll) ? drop_n(arguments(ll), n-1) : drop_n(cdr(ll), n-1)
     end
 end
 @inline drop_n(ll::Union{Tuple, AbstractArray}, n) = drop_n(LL(ll, 1), n)
@@ -218,10 +218,12 @@ macro matchable(expr)
     get_name(e::Expr) = (@assert(e.head == :(::)); e.args[1])
     fields = map(get_name, fields)
     quote
+        # TODO: fix this to be not a call. Make pattern matcher work for these
         $expr
-        SymbolicUtils.istree(::$name) = true
+        SymbolicUtils.head(::$name) = $name
         SymbolicUtils.operation(::$name) = $name
         SymbolicUtils.arguments(x::$name) = getfield.((x,), ($(QuoteNode.(fields)...),))
+        SymbolicUtils.children(x::$name) = [SymbolicUtils.operation(x); SymbolicUtils.children(x)]
         Base.length(x::$name) = $(length(fields) + 1)
         SymbolicUtils.similarterm(x::$name, f, args, type; kw...) = f(args...)
     end |> esc
@@ -229,7 +231,7 @@ end
 
 """
   node_count(t)
-Count the nodes in a symbolic expression tree satisfying `istree` and `arguments`.
+Count the nodes in a symbolic expression tree satisfying `iscall` and `arguments`.
 """
-node_count(t) = istree(t) ? reduce(+, node_count(x) for x in arguments(t), init = 0) + 1 : 1
+node_count(t) = iscall(t) ? reduce(+, node_count(x) for x in arguments(t), init = 0) + 1 : 1
 
diff --git a/test/interface.jl b/test/interface.jl
index d98d97328..af83fc89f 100644
--- a/test/interface.jl
+++ b/test/interface.jl
@@ -1,5 +1,5 @@
 using SymbolicUtils, Test
-import SymbolicUtils: istree, issym, operation, arguments, symtype
+import SymbolicUtils: iscall, issym, operation, arguments, symtype
 
 issym(s::Symbol) = true
 Base.nameof(s::Symbol) = s
diff --git a/test/runtests.jl b/test/runtests.jl
index 004b26b7d..3098331ab 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -33,6 +33,7 @@ else
     include("code.jl")
     include("cse.jl")
     include("interface.jl")
-    include("fuzz.jl")
+    # Disabled until https://github.com/JuliaMath/SpecialFunctions.jl/issues/446 is fixed
+    # include("fuzz.jl")
     include("adjoints.jl")
 end