Allows one to have parts of the view that are alternatives to each other.
The xml prefix of the elements that are alternatives to each other
The xml markup containing the alternate view elements
A Seq[String] of the labels of the elements that are alternatives to each other
A Pair/Tuple2 containing the index in the Seq[String] to use, and a NodeSeq=>NodeSeq to process that element.
Can be used to support a bind-chaining syntax, for use with multiple prefixes.