diff --git a/src/JunctionTrees.jl b/src/JunctionTrees.jl
index 1c756b4..e99a8a2 100644
--- a/src/JunctionTrees.jl
+++ b/src/JunctionTrees.jl
@@ -30,7 +30,6 @@ export SupernodeTree, width, height, seperators, supernode, order
 include("junction_trees/orders.jl")
 include("junction_trees/elimination_algorithms.jl")
 include("junction_trees/ordered_graphs.jl")
-include("junction_trees/abstract_trees.jl")
 include("junction_trees/trees.jl")
 include("junction_trees/postorder_trees.jl")
 include("junction_trees/elimination_trees.jl")
diff --git a/src/junction_trees/abstract_trees.jl b/src/junction_trees/abstract_trees.jl
deleted file mode 100644
index 618e23d..0000000
--- a/src/junction_trees/abstract_trees.jl
+++ /dev/null
@@ -1,25 +0,0 @@
-# A rooted tree.
-abstract type AbstractTree end
-
-
-# Compute a postordering of tree's vertices.
-function postorder(tree::AbstractTree)
-    n = length(tree)
-    order = Vector{Int}(undef, n)
-    index = Vector{Int}(undef, n)
-    
-    for node in PreOrderDFS(IndexNode(tree))
-        order[n] = node.index
-        index[node.index] = n
-        n -= 1
-    end
-    
-    Order(order, index)
-end
-
-
-# Get the height of a tree.
-function height(tree::AbstractTree)
-    n = length(tree)
-    maximum(map(i -> level(tree, i), 1:n))
-end
diff --git a/src/junction_trees/elimination_trees.jl b/src/junction_trees/elimination_trees.jl
index ee76266..f6cbe18 100644
--- a/src/junction_trees/elimination_trees.jl
+++ b/src/junction_trees/elimination_trees.jl
@@ -1,6 +1,6 @@
 # An ordered graph (G, σ) equipped with the elimination tree T of its elimination graph.
 # Nodes i in T correspond to vertices σ(i) in G.
-struct EliminationTree{T <: AbstractTree}
+struct EliminationTree{T <: Union{Tree, PostorderTree}}
     tree::T              # elimination tree
     ograph::OrderedGraph # ordered graph
 end
diff --git a/src/junction_trees/postorder_trees.jl b/src/junction_trees/postorder_trees.jl
index 450282c..319454b 100644
--- a/src/junction_trees/postorder_trees.jl
+++ b/src/junction_trees/postorder_trees.jl
@@ -1,5 +1,5 @@
 # A postordered rooted tree.
-struct PostorderTree <: AbstractTree
+struct PostorderTree
     parent::Vector{Int}           # parent
     children::Vector{Vector{Int}} # children
     level::Vector{Int}            # level
@@ -36,7 +36,7 @@ end
 
 
 # Postorder a tree.
-function PostorderTree(tree::AbstractTree, order::Order)
+function PostorderTree(tree::Tree, order::Order)
     n = length(tree)
     parent = collect(1:n)
     
@@ -72,6 +72,12 @@ function isdescendant(tree::PostorderTree, i::Integer, j::Integer)
 end
 
 
+# Get the height of a tree.
+function height(tree::PostorderTree)
+    maximum(tree.level)
+end
+
+
 ##########################
 # Indexed Tree Interface #
 ##########################
diff --git a/src/junction_trees/trees.jl b/src/junction_trees/trees.jl
index 0b26371..fcc2069 100644
--- a/src/junction_trees/trees.jl
+++ b/src/junction_trees/trees.jl
@@ -1,5 +1,5 @@
 # A rooted tree.
-struct Tree <: AbstractTree
+struct Tree
     root::Int                     # root
     parent::Vector{Int}           # parent
     children::Vector{Vector{Int}} # children
@@ -35,6 +35,22 @@ function Base.length(tree::Tree)
 end
 
 
+# Compute a postordering of tree's vertices.
+function postorder(tree::Tree)
+    n = length(tree)
+    order = Vector{Int}(undef, n)
+    index = Vector{Int}(undef, n)
+    
+    for node in PreOrderDFS(IndexNode(tree))
+        order[n] = node.index
+        index[node.index] = n
+        n -= 1
+    end
+    
+    Order(order, index)
+end
+
+
 ##########################
 # Indexed Tree Interface #
 ##########################