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) |