greek: Optimise a bit
greek: Optimise a bit

Turn `substring ... among ... or substring ... among ...` into a
single `substring ... among ...` in cases where it is trivial to
do so.

snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball