#!/usr/bin/perl

   use strict;

   my %donts;
   my %calls;

   while (chomp (my $line = <DATA>)) {

      my ($mom, $kid) = split(' ', $line);
   
      # First time moms
      if (!defined($calls{$mom})) {
         $calls{$mom} = {};
      }

      $calls{$mom}->{$kid}++;
      print "$mom calls $kid\n";

      # First time kids
      if (!defined($donts{$kid})) {

         $donts{$kid} = {};

         # Kids shouldn't call themselves
         $donts{$kid}->{$kid}++;
         print "$kid better not call $kid\n";
      }

      # Kids shouldn't call their mom
      $donts{$kid}->{$mom}++;

      # Kids shouldn't call their mom's moms
      for my $nono (keys(%{$donts{$mom}})) {

         $donts{$kid}->{$nono}++;
         print "$kid better not call $nono\n";

         # See if they do
         if ($calls{$kid}->{$nono}) {
            print "uh-oh\n";
            exit;
         }
      }
   }

   print "whew!\n";

__DATA__
aaa bbb
bbb ccc
bbb ddd
ccc ddd
ddd aaa
ddd eee