The code had a nested use of the follow() function that could cause +=+
and -=- to register as ++ and --. The first follow() to execute could
consume a character and match and then the second follow() could consume
another character and match. For example i-=-10 would result in a syntax
error and i-=- would decrement i.
(imported from plan9port commit f1dd3f065a97f57bf59db2e3284868e181734159)
450ec696ee