changd semantics of @rel(f)

it now means "like the effect of all apply methods of f". simplifies
the common case where the relative effect refers to the apply method
of a function.

guard against erroneous trees by just returning the bottom effect.
erroneous trees might even appear in "computeEffect" when there's no
error message at the end of the compilation! due to "silent" type
checks, such as in typedApply.