function operate(op, left, right){
  utils.assertType(op, 'string', 'op');
  utils.assertPresent(left, 'left');
  utils.assertPresent(right, 'right');
  return left.operate(op.val, right);
}
Perform `op` on the `left` and `right` operands.

@param {String} op
@param {Node} left
@param {Node} right
@return {Node}
@api public