sub _add_prereq {
  my ($mm_key, $module, $version_or_range) = @_;
  warn "$module already exists in $mm_key -- need to do a sane metamerge!"
    if exists $WriteMakefileArgs{$mm_key}{$module}
      and $WriteMakefileArgs{$mm_key}{$module} ne ($version_or_range || 0);
  warn "$module already exists in FallbackPrereqs -- need to do a sane metamerge!"
    if exists $FallbackPrereqs{$module} and $FallbackPrereqs{$module} ne ($version_or_range || 0);
  $WriteMakefileArgs{$mm_key}{$module} = $FallbackPrereqs{$module} = $version_or_range || 0;
  return;
}
