VBAC, section 8.22.11

     Assigner Call Validity rule

     An Assigner_call of the form
        target := source
     where target and source are expressions, is valid if
     and only if it satisfies the following conditions:

        1 source is compatible with target.

        2 The Equivalent Dot Form of target is a qualified
          Object_call whose feature has an assigner
          command.

        3 If the query of the Object_call is stable, then
          the type of the source is attached.