_transform = function _transform(node) {
          if (node.isSymbolNode && matches.placeholders.hasOwnProperty(node.name)) {
            return matches.placeholders[node.name].clone();
          } else {
            return node.map(_transform);
          }
        }
Returns a simplfied form of node, or the original node if no simplification was possible.

@param  {ConstantNode | SymbolNode | ParenthesisNode | FunctionNode | OperatorNode} node
@return {ConstantNode | SymbolNode | ParenthesisNode | FunctionNode | OperatorNode} The simplified form of `expr`, or the original node if no simplification was possible.