VUEX, chapter 22 (Type checking), page 368

     A call appearing in a class `C',  having  `fname'  as
     the  feature of the call, is export-valid for a class
     `D' if  and  only  if  it  satisfies  either  of  the
     following two conditions.

        1 The call is an Unqualified_call and  `fname'  is
          the final name of a feature of `C'.

        2 The call has at least one dot, `D' has a feature
          of  name  `fname' which is available to `C', and
          the call's target is either a  valid  entity  of
          `C'  or  (recursively)  a  call which is export-
          valid for `D'.