code: Rule IfieldMeter() {
  		return Sequence(String("[M:"),
  				ZeroOrMore(WSP()).suppressNode(),
  				TimeSignature(),
  				String("]"))
  			.label(IfieldMeter);
  	}
docs: ifield-meter ::= %5B.4D.3A *WSP meter %5D<p>
  e.g. <tt>[M:C|] [M: 2+3/8]</tt>