File		= {Production}.
Production	= Nonterminal [ String ] "=" Term "." .
Term		= Factor / "|" .		# alternative
Factor		= ExtAtom + .			# sequence
ExtAtom		= Atom
		| Atom "/" Atom			# repetion through Atom
		| Atom "+".			# at least one repetion
Atom		= Nonterminal
		| String			# terminal string
		| "(" Term ")"
		| "[" Term "]"			# an optional Term
		| "{" Term "}"			# zero or more repetions
		.
String		= "\"" { character } "\"" .
Nonterminal	= letter { letter | digit | "_" } .
character	= "\\" charesc.