-PreSequence ::= NamedElements => "ps"
- | PreSequence !wp ^"/" !ws e
- | PreSequence ^"->" e /ws
- | PreSequence ^"=>" ("[]"|"()"|word|Quoted) /ws
-Sequence ::= PreSequence => "psx"
+Sequence = psx:: PreSequence
+ | Sequence !ws ^"&" !ws Elements
+ | Sequence !ws ^"&~" !ws Elements