Write a function topological_ordering that takes the location of a text file containing the description of a directed acyclic graph $$\mathcal{G}$$ in the form of an adjacency list. Each line of the file contains the label (an integer) of a starting node, a space, the characters ->, another space and a comma-separated list containing the lexicographically sorted labels (integers) of all ending nodes connected to that starting node. The function must return a topological ordering of graph $$\mathcal{G}$$, represented as tuple of node labels.

Example

In the following interactive session, we assume the text file data01.txt¹ to be located in the current directory.