Ken Kundert writes:

The rule is you cannot give unit without a scale factor, and the unity scale factor is _, so if you wanted to say 1 mol you would use 1_mol. 1mol means one milli ol. These look a little strange, but that is because the use they unit scale factor, which is the one that is currently not in heavy use.

One reason I like Python is that it has relatively few of these irregularities and ambiguities (in comparison to other languages and non-programming contexts with similar usage). For me, that counts against this proposal. BTW, where is "_" used as the unit scale prefix?

I suggest that we do not support the h (=100), da (=10), d (=0.1), or c (=0.01) scale factors.

I don't think it's reasonable to exclude those. Around me, cm, dB, and ha (centimeters, decibels, and hectares) are in common use. What happened to "support with a capital S"? I don't speak for anybody but myself, but I think this proposal has gotten less interesting/acceptable with each post. I'm going wait and see if the "units are types" approach goes anywhere. I think it's probably the only one that has wings, but that's because it requires no change to the language.