Makes a byX function, like byStat or byClass
A method for finding your stat, or class, or whatever X is in this context
A function akin to byStat or byClass; it accepts an object that either is "complete" in the sense that it has a key for every conceivable value, or contains a default parameter. If an inappropriate input is provided, returns undefined.
default
Optional
Makes a byX function, like byStat or byClass