added type xrule (from sextension.ML); removed old 'extend'; removed added syn_ext_const_names, syn_ext_trfuns (now in syn_ext.ML); various minor changes;
added type xrule (from sextension.ML);
removed old 'extend';
removed added syn_ext_const_names, syn_ext_trfuns (now in syn_ext.ML);
various minor changes;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle