ref: 2bfb79be604c68b7684b515f3be3388fecfcf1f4
dir: pprolog/TODO
* Stop comparing strings all the time * Stop copying the entire goal stack into every choicepoint * Stop creating choicepoints when it is not needed * How to implement builtins nicely? * Right now we copy and allocate a lot, but almost never free stuff. * Many builtins should really throw an error, but they just fail for now. * Exceptions (throw, catch) * Modules