psy ::= ps
| ps wp ^"/" ws e
psx ::= psy => "psy"
| e "<-" psy /ws => "psyl"
| psy "->" e /ws => "psyr"
| e "<-" psy "->" e /ws => "psylr"
psy ::= ps
| ps wp ^"/" ws e
psx ::= psy => "psy"
| e "<-" psy /ws => "psyl"
| psy "->" e /ws => "psyr"
| e "<-" psy "->" e /ws => "psylr"