			/* Simplification */

simplify(n) => n where numberp(n) = true.
simplify([op, e1, e2]) => op(simplify(e1), simplify(e2)).

add(x,y) => x+y.
times(x,y) => x*y.
sub(x,y) => x-y.
quo(x,y) => x div y.
