First apply a fixed pickler/unpickler, then a 2. one
If the first fails during unpickling, the whole pickler fails. This
can be used to check some properties of the input, e.g. whether a
given fixed attribute or a namespace declaration exists
(
xpAddFixedAttr,
xpAddNSDecl) or to filter the input,
e.g. to ignore some elements or attributes (
xpFilterCont,
xpFilterAttr).
When pickling, this can be used to insert some fixed XML pieces, e.g.
namespace declarations, class attributes or other stuff.