diff --git a/src/stdlib/models/FinSets.jl b/src/stdlib/models/FinSets.jl
index 8d9aa3ea..8bf8d303 100644
--- a/src/stdlib/models/FinSets.jl
+++ b/src/stdlib/models/FinSets.jl
@@ -27,6 +27,7 @@ end
   compose(f::Vector{Int}, g::Vector{Int}) = g[f]
 
   dom(f::Vector{Int}) = length(f)
+  codom(::Vector{Int}; context) = context[:codom]
 end
 
 end
diff --git a/src/stdlib/models/Op.jl b/src/stdlib/models/Op.jl
new file mode 100644
index 00000000..cbb6da38
--- /dev/null
+++ b/src/stdlib/models/Op.jl
@@ -0,0 +1,37 @@
+module Op 
+
+export OpC, op
+
+
+using ....Models
+using ...StdTheories
+using StructEquality
+
+
+@struct_hash_equal struct OpC{ObT, HomT, C<:Model{Tuple{ObT, HomT}}} <: Model{Tuple{ObT, HomT}}
+  cat::C
+end
+
+op(c) = OpC(c)
+
+using .ThCategory
+
+rename(::Nothing, ::Dict{Symbol,Symbol}) = nothing
+rename(nt::NamedTuple, d::Dict{Symbol,Symbol}) = 
+  NamedTuple(get(d, k, k) => v for (k, v) in pairs(nt))
+
+
+@instance ThCategory{ObT, HomT} [model::OpC{ObT, HomT, C}] where {ObT, HomT, C} begin
+  Ob(x::ObT) = Ob[model.cat](x)
+  Hom(x::HomT, d::ObT, cd::ObT) = Hom[model.cat](x, cd, d)
+  id(x::ObT) = id[model.cat](x)
+  dom(f::HomT; context) = 
+    codom[model.cat](f; context=rename(context, Dict(:dom=>:codom, :codom=>:dom)))
+  codom(f::HomT; context) = 
+    dom[model.cat](f; context=rename(context, Dict(:dom=>:codom, :codom=>:dom)))
+  compose(f::HomT, g::HomT; context=nothing) =
+    compose[model.cat](g, f; 
+            context=rename(context, Dict(:a=>:c, :c=>:a, :b=>:b)))
+end
+
+end # module 
diff --git a/src/stdlib/models/module.jl b/src/stdlib/models/module.jl
index b7613e4e..328c7fe6 100644
--- a/src/stdlib/models/module.jl
+++ b/src/stdlib/models/module.jl
@@ -5,12 +5,14 @@ using Reexport
 include("FinSets.jl")
 include("FinMatrices.jl")
 include("SliceCategories.jl")
+include("Op.jl")
 include("Nothings.jl")
 include("GATs.jl")
 
 @reexport using .FinSets
 @reexport using .FinMatrices
 @reexport using .SliceCategories
+@reexport using .Op
 @reexport using .Nothings
 @reexport using .GATs
 
diff --git a/test/stdlib/models/FinSets.jl b/test/stdlib/models/FinSets.jl
index 04f67287..7887388e 100644
--- a/test/stdlib/models/FinSets.jl
+++ b/test/stdlib/models/FinSets.jl
@@ -4,7 +4,7 @@ using GATlab, Test
 
 using .ThCategory
 
-@withmodel FinSetC() (Ob, Hom, id, compose, dom) begin
+@withmodel FinSetC() (Ob, Hom, id, compose, dom, codom) begin
   @test Ob(0) == 0
   @test_throws TypeCheckFail Ob(-1)
   @test_throws TypeCheckFail Hom([1,5,2], 3, 4)
@@ -13,6 +13,7 @@ using .ThCategory
   @test id(2) == [1,2]
   @test compose([5], [1,1,1,3,2]) == [2]
   @test dom([5]) == 1
+  @test codom([5]; context=(codom=10,)) == 10
 end
 
 end
diff --git a/test/stdlib/models/Op.jl b/test/stdlib/models/Op.jl
new file mode 100644
index 00000000..06295f1f
--- /dev/null
+++ b/test/stdlib/models/Op.jl
@@ -0,0 +1,21 @@
+"""Same as FinSetC tests but with all doms/codoms reversed"""
+module TestOp
+
+using GATlab, Test
+
+using .ThCategory
+
+@withmodel op(FinSetC()) (Ob, Hom, id, compose, dom, codom) begin
+  @test Ob(0) == 0
+  @test_throws TypeCheckFail Ob(-1)
+  @test_throws TypeCheckFail Hom([1,5,2], 4, 3)
+  @test Hom(Int[], 4, 0) == Int[]
+
+  @test id(2) == [1,2]
+  @test compose([1,1,1,3,2], [5]) == [2]
+  @test compose([1,1,1,3,2], [5]; context=(;)) == [2]
+  @test codom([5]) == 1
+  @test dom([5]; context=(dom=10,)) == 10
+end
+
+end # module
diff --git a/test/stdlib/models/tests.jl b/test/stdlib/models/tests.jl
index 6e7e807a..6555e913 100644
--- a/test/stdlib/models/tests.jl
+++ b/test/stdlib/models/tests.jl
@@ -3,6 +3,7 @@ module TestStdModels
 include("FinSets.jl")
 include("FinMatrices.jl")
 include("SliceCategories.jl")
+include("Op.jl")
 include("Nothings.jl")
 include("GATs.jl")