No easier to implement than arbitrary computational macros, but is:
 -- More powerful since it can access semantic information inferred by the
    compiler.
 -- Safer, since it automatically enforces hygiene and evaluation order by
    being based on function semantics.

result type matching?
Probably want mechanism for declaring functions flushable and foldable.

defining
arg-constant?
arg-constant-value
arg-type

limited (and singleton?) accessors
types-intersect/type-intersection
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