asserts

[Bibi56]

add "-ea" in VM options in run config for the asserts to work
(in README.md)

I don't know if this needs some rewriting or the code.
But asserts are optimized (at least on some JVMs).
So:
assert begin(gl);
Does... nothing without adding -ea
boolean result = begin(gl);
assert result;
Would be fine with or without the option.