A couple of things on my checklist for mathematical purity are (a) first-class macros and (b) hardcoded builtins. It looks like Bel does have first-class macro

A couple of things on my checklist for mathematical purity are (a) first-class macros and (b) hardcoded builtins. It looks like Bel does have first-class macros. As for (b)... > Some atoms evaluate to themselves. All characters and streams do, along with the symbols nil, t, o, and apply. All other symbols are variable names The definitions of "ev" and "literal" establish that nil, t, o, and apply are in fact hardcoded and unchangeable. Did you consider having them be variables too, which just happen to be self-bound (or bound to distinctive objects)? nil is a bit of a special case because it's also the end of a list, and "(let nil 3 5)" implicitly ends with " . nil"; o might be an issue too (said Tom arguably); but apply and t seem like they could be plain variables. P.S. It looks like you did in fact implement a full numerical tower—complex numbers, made of two signed rational numbers, each made of a sign and a nonnegative rational number, each made of two nonnegative integers, each of which is a list of zero or more t's. Nicely done.