This fixes Mantis bug #528.
compiler/error_msg_inst.m:
Since an inst_name IN may be used nested inside itself, as in
IN(..., IN(...), ...), do not insist that both occurrences of IN
in such cases lead to the insertion of a new entry in the inst name
expansion map.
tests/invalid/html.{m,err_exp}:
The Mantis test case.
tests/invalid/Mmakefile:
Enable the Mantis test case.
