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Would be useful to have a Freeze-Type proclamation. Its primary use would be to say that the indicated type won’t acquire any new subtypes in the future. This allows better open-coding of structure type predicates, since the possible types that would satisfy the predicate will be constant at compile time, and thus can be compiled as a skip-chain of EQ tests.
Of course, this is only a big win when the subtypes are few: the most important case is when there are none. If the closure of the subtypes is much larger than the average number of supertypes of an inferior, then it is better to grab the list of superiors out of the object’s type, and test for membership in that list.
Should type-specific numeric equality be done by EQL rather than =? i.e. should = on two fixnums become EQL and then convert to EQL/FIXNUM? Currently we transform EQL into =, which is complicated, since we have to prove the operands are the class of numeric type before we do it. Also, when EQL sees one operand is a FIXNUM, it transforms to EQ, but the generator for EQ isn’t expecting numbers, so it doesn’t use an immediate compare.