VWID, chapter 23 (Expressions and constants), page 386

     Identifier rule

     An Identifier appearing in an expression as  part  of
     the  text  of a routine `r' in a class `C', either by
     itself or as the target or actual argument of a Call,
     must  be the name of a feature of `C', a local entity
     of `r', or a formal argument of `r'.