VNCB, chapter 13 (Conformance), page 229

     The possible direct  conformance  cases  involving  a
     Bit_type  are the following for any positive integers
     `N' and `P':

        1 BIT` N' conforms directly to `ANY'.

        2 BIT` N' conforms directly to BIT` P' for `N' <= `P'.

          Other than  implied  by  these  rules,  no  type
          conforms  directly to a Bit_type, and a Bit_type
          conforms directly to no type.