Ref Man for my contribution

I'll be happy if this could be proofread before I merge ...
I think just remove "rule", and use `applies $\beta\iota$-reduction.` because there are both β and ι rules, and no special βι rule, and English lacks a modifier for this kind of aggregate construct where you get to factor β-reduction and ι-reduction into βι-reduction, and you have to decide what to do with nouns that are modified by these.