labeled version of an indentedpreorder presentationakin to code fragment could be generated by the new subclass of eulertour shown in code fragment class preorderprintindentedlabeledtour(eulertour) def hook previsit(selfpdpath) label join(str( + for in pathlabels are one-indexed labelp element) print( code fragment subclass of eulertour that produces labeled and indentedpreorder list of tree' elements to produce the parenthetic string representationoriginally achieved with code fragment we define subclass that overrides both the previsit and postvisit hooks our new implementation is given in code fragment class parenthesizetour(eulertour) def hook previsit(selfpdpath) if path and path[- follows sibling so preface with comma printendthen print element print( element)endif has children if not self treeis leaf( )print opening parenthesis printend def hook postvisit(selfpdpathresults)if has children if not self treeis leaf( )print closing parenthesis printendcode fragment subclass of eulertour that prints parenthetic string representation of tree notice that in this implementationwe need to invoke method on the tree instance that is being traversed from within the hooks the public treemethod of the eulertour class serves as an accessor for that tree finallythe task of computing disk spaceas originally implemented in code fragment can be performed quite easily with the eulertour subclass shown in code fragment the postvisit result of the root will be returned by the call to execute class diskspacetour(eulertour) def hook postvisit(selfpdpathresults) we simply add space associated with to that of its subtrees return elementspacesum(resultscode fragment subclass of eulertour that computes disk space for tree