Fixed bug in add_primrec which caused non-informative error message.
Fixed bug in add_primrec which caused non-informative error message.

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