Bug in `new_nac_homs!`

The logic of `new_nac_homs!` is to find, after a deletion $f: X \leftarrowtail X'$, new morphisms which may have arisen due to a NAC $n: L \rightarrow N$ being deactivated.

This is done via finding morphisms $h: N\rightarrow X$ that meet some property, then pulling back $n \cdot h$ along $f$ to obtain the match $L\rightarrow X'$. However, something is awry in defining "some property", since the code presently allows $h$ to map parts of $L$ into deleted parts of $X$, which make the pulled back morphism $L\rightarrow X'$ undefined. So that logic needs to be looked into + improved.