Partial constraint on topologicalSort

[no-longer-on-githu-b]

If the graph contains cycles, then the behavior is undefined.

Should there be a Partial constraint?

An alternative is to make sure the function is total and then change the documentation to:

If the graph contains cycles, an unspecified value is returned.

[vyorkin]

As I understand, the function is total, but if there are cycles in the input graph then the resulting list of keys is wrong, so the latter might be a better option.