lib/ProofGeneral/schemas.xml

adding documentation about the quickcheck_generator command in the IsarRef

<?xml version="1.0"?>
<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <uri resource="pgip_isar.xml" uri="pgip.rnc"/>
  <uri resource="pgip_isar.xml" typeId="RELAX NG"/>
</locatingRules>