We currently only handle "Validity constraint: Proper Declaration/PE
Nesting", but we must detect "Well-formedness constraint: PE Between
Declarations" separately:
> The replacement text of a parameter entity reference in a DeclSep must
> match the production extSubsetDecl.
PEs in DeclSeps are PEs that start with a full markup declaration (or
another PE). These are handled in xmParse{Internal|External}Subset. We
set a flag on these PEs and don't close them implicitly in
xmlSkipBlankCharsPE. This will make unterminated declarations in such
PEs cause a parser error. The PEs are closed explicitly in
xmParse{Internal|External}Subset, the only location where they are
allowed to end.
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