public Prologue sub(PrefixMapping newMappings, String base)
{
// New prefix mappings
PrefixMapping ext = getPrefixMapping() ;
if ( newMappings != null )
ext = new PrefixMapping2(ext, newMappings) ;
// New base.
IRIResolver r = resolver ;
if ( base != null )
r = IRIResolver.create(base) ;
return new Prologue(ext, r) ;