code: function operate(op, left, right){
    utils.assertType(op, 'string', 'op');
    utils.assertPresent(left, 'left');
    utils.assertPresent(right, 'right');
    return left.operate(op.val, right);
  }
docs: Perform `op` on the `left` and `right` operands.
  
  @param {String} op
  @param {Node} left
  @param {Node} right
  @return {Node}
  @api public