Cases: 1. pgroup value is 'true' => 0 2. pgroup value is kind of true => Rubinius will try coerce_to Integer 3. pgroup value if kind of true and pass coerce then Rubinius will check check whether the value is < 0. If yes then an ArgumentError exception is thrown 4. pgroup if kind of false (false, nil) - default behavior (like without pgroup) Fixes #1894
This makes sure that valgrind can properly check whether we overflow or underflow our stacks. Without this it will wrongly complain about accessing stuff outside the stack in fibers.
Github just pushed the contributing guidelines feature: https://github.com/blog/1184-contributing-guidelines