The §Typecheck enumeration in 0012-tail-calls.md presented the
tail-position rule as exhaustive ("The flag is `false` for: ...") but
omitted `Term::If` entirely — even though `verify_tail_positions`
propagates `is_tail` into both `If` branches (ailang-check/src/lib.rs),
and 0012's own motivating example (`if n == 0 then () else loop(n-1)`)
relies on the `else` branch being a tail position.
Restate the rule by principle (a sub-term is in tail position iff it is
the value of the whole expression) and enumerate the actual checker's
arms: `If` branches, `Match` arms, `Seq` right, `Let`/`LetRec`
in-clause, `Lam` body, and the identity/`Loop` propagators; every
argument position (incl. `If` condition, `Recur`/`New` args) is
non-tail. Matches the present `verify_tail_positions` walker.
Brummel
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