Fix for #923

[rhendric]

This issue involved a lexer flag that was set in a for comprehension and
never cleared when the comprehension closed. The fix consists of
clearing all lexer flags at the current delimiter depth every time a
close delimiter is encountered. (It may be that this makes some
occurrences of @fset _ false elsewhere in the lexer redundant; to be
safe, I didn't attempt to remove any of them in this commit.)

[gkz]

Thanks!