The `procedure-specialize` function is the identity function, but it
provides a hint to the JIT to compile the body of a closure
specifically for the values in the closure (as opposed to compiling
the body generically for all closure instances).
This hint is useful to the contract system, where a predicate
is coerced to a projection with
(lambda (p?)
(procedure-specialize
(lambda (v)
(if (p? v)
v
....))))
Specializing the projection to a given `p?` allows primitive
predicates to be JIT-inlined in the projection's body.
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