https://github.com/rwtema/Monk-Mod/blob/master/src/main/java/com/rwtema/monkmod/advancements/criteria/StatePredicate.java#L7

Minor quibble, but browsing over the source I was slightly confused that StatePredicate wasn't annotated with https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html