Exercises.

1. Define the relation niece-or-nephew in Prolog.

2. Define the relation cousin in Prolog.

3. Major Projects:
   Modify the interpreter to delay any negated atom until its variables are all bound (if ever).

   Add predicates `int' and `is' to Prolog-S. `int(N)' succeeds if and only if N is bound to an integer. `is(X,E)' or `X is E' succeeds if and only if E is an arithmetic expression containing only operators, numbers and variables bound to numbers and in that case X is unified with the expression's value.

   Use `int' and `is' to write a simplifier in Prolog to go with the symbolic differentiator.