sub same_sign {
    my ($x,$y) = @_;
    if ( undef($x) or undef($y)) {
       return 0;           # "undef" is never same-sign
    }
    if (   ( ($x >= 0) and ($y >= 0) )
        or ( ($x <  0) and ($y <  0) ) )
    {
       return 1;
    }
    else 
    {
       return 0;
    }
  }