PcsBuilder fails to add child list of leaf nodes

[slarse]

This is a serious bug. The PcsBuilder fails to add the empty child lists for leaf nodes. That is to say, a leaf node v should have PCS(v, START, END), but they don't get that. A consequence is that deletions that remove the entire child list aren't detected. For example, changing return 1; to return; goes unnoticed as the return statement's child list is entirely removed, but the empty child list is not represented in the PCS set.

This also means that the pseudocode in the thesis is incorrect. Oopsie.