These are not that important, because 'CpyTree'
occurs only on compressed paths, and compression
only happens at the end.
But having them for completeness and making the
functions total is not going to harm.
It is present in the thesis' Agda version but
also dropped in the Haskell version, which was
the basis for the 'gdiff' library.
As explained in the thesis, patchL is not total
but this clause was strictly necessary to work