-* We need to be careful if we just look at f's arity. Currently (Dec07),
- f's arity is visible in its own RHS (see Note [Arity robustness] in
- SimplEnv) so we must *not* trust the arity when checking that 'f' is
- a value. Instead, look at the unfolding.
+* Note [Arity care]: we need to be careful if we just look at f's
+ arity. Currently (Dec07), f's arity is visible in its own RHS (see
+ Note [Arity robustness] in SimplEnv) so we must *not* trust the
+ arity when checking that 'f' is a value. Otherwise we will
+ eta-reduce
+ f = \x. f x
+ to
+ f = f
+ Which might change a terminiating program (think (f `seq` e)) to a
+ non-terminating one. So we check for being a loop breaker first.