VUAR, chapter 22 (Type checking), page 369

     An export-valid call of target type `ST'  and feature
     `fname' appearing in a class `C'  where it  denotes a
     feature  `sf'  is  argument-valid  if  and only if it
     satisfies the following conditions:


       1 The number of  actual  arguments  is  the same as
         the number of formal arguments declared for `sf'.

       2 Every actual  argument of the call is  compatible
         with the corresponding formal argument of `sf'.