The reason the mapOver function doesn't have a case for the absence of a path passed is because it always expects to traverse a MatchExpression from the top. The path will get initialized to "" and then continuously get appended to/change as we traverse the nodes of the MatchExpression. The function as written has a few risks:

• If we pass in a null MatchExpression, this function will crash
• We may in the future want to differentiate between the absence of a path (using boost::optional) and ""