An example of how to use Carp::Diagnostics.
The default rule of a FirstAndOnlyOneOnDisk META_RULE must be registrated before the META_RULE definiton. Here is an example of declaration:
AddRule 'c_o', [ '*/*.o' => '*.c' ] => \&C_Builder ;
AddRule 'cpp_o', [ '*/*.o' => '*.cpp' ] => \&CPP_Builder ;
AddRule [META_RULE], 'o_cs_meta' =>
[\&FirstAndOnlyOneOnDisk, ['cpp_o', 'c_o' ], 'c_o'] ;
^- slave rules -^ ^-default