I do understand, but that's only true for [0Oo], and does not help with [1Il]. In either case, the risk of human confusion is removed by transliteration in pars

I do understand, but that's only true for [0Oo], and does not help with [1Il]. In either case, the risk of human confusion is removed by transliteration in parser code. But on the parser implementation side, I need to remember that numeric 0 maps to lowercase o, but lowercase l maps to numeric 1. My thought is that it would be simpler to only remember one rule: numeric chars are canonical, lookalikes are dropped. Clearly, not a big deal -- you write it once, correctly, and move on (or try to!). That "once" has apparently stuck in my head, and now a few years later I still wonder about the reason for the design choice. :)