JOIN

• input: C1 and C2 (collections).
• output: C1' and C2'.
• preconditions: C1 and C2 are both defined, and are different collections.
• postconditions: C1' is the union of the two collections (i.e. it contains all the elements in C1 and C2). C2' is undefined.

MAP

• input: C (a collection), P (a procedure), D extra data parameter for P.
• output: determined by P.
• preconditions: C is defined. P takes two parameters: the first is of type pointer to `collection element', and the second is D.
• postconditions: P has been called with every element of C.