From @mtlcl410's independent run of this PR:
- a second subset of the same font can be re-encoded with `/Differences`, so
"-l" has different codes from the first subset's; the line still joins,
reads in coding order, and editing it replaces every run, including the
re-encoded subset's;
- `ABCDE+Helvetica-Bold` has a five-letter prefix, not the six-letter subset
tag of ISO 32000-2 §9.6.4, so it stays a different font from
`Helvetica-Bold` and the runs stay apart.
The first case (and its edit) fails before the subset fix; the second guards
the tag-length rule against stripping any `+` prefix.
Aman Mehtar
1d76fcbc3b