VGCS, chapter 18 (Creating objects), page 288

     A Creation instruction is system-valid if and only if
     it satisfies one of the following two conditions:

        1 The creation type is explicit (in  other  words,
          the  instruction  begins with ! T ! ... for some
          type `T').

        2 The creation type is implicit (in  other  words,
          the  instruction  begins  with  !!...) and every
          possible dynamic type `T'  for  `x',  with  base
          class  `C',  satisfies  conditions 1 to 6 of the
          Creation Instruction rule (page 0).  In applying
          conditions  5  and  6,  the feature of the call,
          `f', must be replaced by its version in `C'.