gistfile1.txt

Works:

        (1 ..^ +@.points).grep({
                                    my $range = (@.points[$_ - 1][0] minmax @.points[$_][0]);
                                    $u ~~ $range;
                                }).map({ self.intersection-info($_, $u); });

Doesn't work:

        (1 ..^ +@.points).grep({
                                    $u ~~ (@.points[$_ - 1][0] minmax @.points[$_][0]);
                                }).map({ self.intersection-info($_, $u); });

trace:

Cannot modify readonly value
  in '&infix:<=>' at line 1
  in <anon> at line 1698:CORE.setting  # this is at_pos
  in '&infix:<=>' at line 1
  in <anon> at line 1532:CORE.setting  # this is Any.minmax
  in 'Any::minmax' at line 1
  in 'infix:<minmax>' at line 596:CORE.setting
  in <anon> at line 183:lib/loop.pm
  in 'Block::ACCEPTS' at line 5912:CORE.setting
  in <anon> at line 1442:CORE.setting
  in <anon> at line 1
  in main program body at line 1