Return the inverse of a mapping document.
The input mapping document.
A mapping document is a d:fileset document that maps files from @original-href to @href and anchors from @original-id to @id.
d:fileset
@original-href
@href
@original-id
@id
The inverse mapping document.
Applying px:fileset-invert twice yields the input document again.
px:fileset-invert