Math. Description of Methods: Domain and Range of the Method's function
Example: the POP operation on a stack can be described by a mathematical function:
fPOP: stacks ? elements ? stacks
stacks = the set of all possible stacks (according to this ADT).
elements = the set of all acceptable elements in our stacks (e.g., integers).
Because the operation produces an element and it causes a change to the stack, the range of fPOP is a cartesian product.