The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
33. Indeclinable nouns, infinitives, terms or phrases
used as nouns, and words quoted merely for their form, are neuter: -
- fas, right; nihil, nothing; gummí, gum; scire tuum, your knowledge (to know);
triste vale, a sad farewell;
hoc ipsum dia, this very