sub gen_triangles {
  my $max = shift || 10;
  my @set;
  my($x,$y,$z)=(1,1,1);
  while($z<=$max && $x<=$max){
    push @set, [$z, $y, $x] if $x**2 == $y**2 + $z**2;
    if($y>$z){$z++}else{if($x>$y){$y++;$z=1}else{$x++;$y=$z=1}}
  }
  return \@set;
}