code: function ( nodes, replacements ) {
  			var index = parseInt( nodes[0], 10 );
  
  			if ( index < replacements.length ) {
  				// replacement is not a string, don't touch!
  				return replacements[index];
  			} else {
  				// index not found, fallback to displaying variable
  				return '$' + ( index + 1 );
  			}
  		}
docs: Return escaped replacement of correct index, or string if
  unavailable. Note that we expect the parsed parameter to be
  zero-based. i.e. $1 should have become [ 0 ]. if the specified
  parameter is not found return the same string (e.g. "$99" ->
  parameter 98 -> not found -> return "$99" ) TODO throw error if
  nodes.length > 1 ?
  
  @param {Array} nodes One element, integer, n >= 0
  @param {Array} replacements for $1, $2, ... $n
  @return {string} replacement