Question 6: Parsing [4]

Write a lisp function that takes a lisp expression as a parameter, and returns true iff the expression is a valid expression using only integers and binary operators + and -. 

   Examples:
        (- 100 12) is valid
        (+ (+ 10 20) (- 5 1)) is valid
        (+ 3 (+ (+ 4 5) 6)) is valid
        (* 100 12) is not valid (invalid operand)
        (+ 1 2 3) is not valid (more than two operands)
        (+ 4) is not valid (less than two operands)
        (+ x 6) is not valid (non-integer operand)