null(nil) = true                     // nil refer for empty tree

null(fork(e, T, T'))= false   //  e : element , T and T are two sub tree leaf(fork(e, nil, nil)) = true

leaf(fork(e, T, T')) = false if not null(T) or not null(T')

leaf(nil) = error

left(fork(e, T, T')) = T

left(nil) = error

right(fork(e, T, T')) = T' right(nil) = error

Contents (fork (e, T, T')) = e contents (nil) = error

Consider the definition of Tree (ADT). A way to think of a binary tree is that this is either empty (nil) or have an element and two sub trees that are themselves binary trees. Fork operation joins two sub trees along a parent node and generates another Binary tree. It might be noted that a tree containing a single leaf is described to be of height 1.

Definition: A tree is connected, acyclic graph