code: Rule BarStaves() {
  		return FirstOf(
  					Sequence(ZeroOrMore(WSP()), '|', ZeroOrMore(WSP())),
  					OneOrMore(WSP())
  				).label(BarStaves);
  	}
docs: bar-staves ::= (*WSP "|" *WSP) / 1*WSP
  <p>
  <tt>|</tt> to not bar