Homework problems.
1. A function, (concat X Y), that will concatenate two lists, X and Y.
2. A function, (del X L), that will delete a scalar, X, from a list, L.
3. A function, (mean L), that returns the average value of a list, L.
4. A function, (compare X Y), that compares two lists, X and Y, and
returns a true if the are equal, or else a nil.
5. A function, (reverse X Y), which reverses the list X onto the list Y
and returns the result. If Y is nil, then the result should be the
reverse of X.
6. A function, (palindrom X), which return a t or nil if the list X
is a palindrom or not.
7. A function, (uniq X), that returns a list that is composed of the
members of X, such that no element of X appears in the result more
than once.
8. A function, (uconc X Y), which produces a uniq concatenation of X
and Y.
9. A function, (diff X Y), which produces the members of X that are not
in Y and the members of Y that are not in X.
10. A function, (gcd X Y), which returns the greatest common denominator
of X and Y using Euclids algorithm.