+NonTerminal = NonTerminal:: Word ws "=" ws RHS
+ | DropNT:: Word "!" ws "=" ws RHS
+ | Colons:: Word "::" ws "=" ws RHS
+ | Word "*" ws ^"=" ws RHS
+ | Word "*/" Word ws ^"=" ws RHS
+
+RHS:: = ("|":: Sequence +/ (ws "|" ws)) +/ (ws ">" ws)
+
+Elements:: = e*/ws
+
+PreSequence = Elements
+ | (Quoted|Word) ^"::" PreSequence /ws
+ > PreSequence ^"/" e /ws
+ | PreSequence ^"->" e /ws
+
+Sequence = PreSequence
+ | Sequence ^"&" Elements /ws
+ | Sequence ^"&~" Elements /ws
+
+e = e ^"!"
+ > (Quoted|Word) ^":" e
+ > NonTerminalReference:: Word
+ | Literal:: Quoted
+ | ^"()"
+ | ^"{" PreSequence "}" /ws
+ | ^"[" Range* "]"
+ | e ^"++" /ws -> ~[/]
+ | e ^"+" /ws -> ~[+]
+ | e ^"++/" e /ws
+ | e ^"+/" e /ws
+ | e ^"**" /ws -> ~[/]
+ | e ^"*" /ws -> ~[*]
+ | e ^"**/" e /ws
+ | e ^"*/" e /ws
+ | e ^"?" /ws
+ | ^"^" Quoted
+ | "(" Word ^")"
+ > ^"(" RHS ")" /ws
+ | ^"~" e
+ | ^"\\{"
+ | ^"\\}"
+
+Word:: = [.a-zA-Z0-9_]++
+Quoted:: = "\"" (~[\"\\] | escaped)+ "\""
+ | "\"\""
+
+Range:: = ec
+ | ec "-" ec
+
+ec = ~[\-\]\\]