VUCS, chapter 22 (Type checking), page 367

     Consider a single-dot call with target `x', appearing
     in a class `C'. Let `S' be the type of `x'.  Then:

        1 The call is class-valid if  it  is  export-valid
          and argument-valid for `S'.

        2 The call is system-valid if for any element  `D'
          of  the  dynamic  class set of `x' it is export-
          valid and argument-valid for `D'.