method display
  {
    iterate 
    { 
      print $.grid[$^x][$^y] ?? '+' :: '.';
      print "\n" if ($^x == $.max);
         # print each row on own line
    }
    print "\n"; # blank line follows grid
    print "Turn $(++$.count), press enter to continue"
    print " or ctl-c to quit";
    <$*IN>;
    .calculate();
  } # end method display