informal methods

What’s a tree? First define a sequence or length n> 0 as a total map s: {1,.. n}→ X. Let Nil be the empty sequence.  If s is a sequence of length n, and x is an element, then u

What’s a linked list?  Let V be a set of values and A be a set of “addresses”. Say a map f is a “linked list element” compatible with values V  and addresses A if and only if f(value)∈ V