code: Rule IfieldTempo() {
  		return Sequence(String("[Q:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				FirstOfS(Tempo(), TempoText()),
  				ZeroOrMore(WSP()).suppressNode(),
  				OptionalS(FirstOfS(Tempo(), TempoText())),
  				String("]"))
  			.label(IfieldTempo);
  	}
docs: ifield-tempo ::= %5B.%51.%3A *WSP tempo %5D<p>
  e.g. <tt>[Q:1/4=120]</tt>