VNCP, chapter 14 (Conformance and convertibility), page xxx

     Conversion procedure rule

     It is valid for a creation procedure cp of a class C, with current
     type CT, to be a conversion procedure if and only if it satisfies the
     following four conditions for every type SOURCE listed in its
     Conversion_types:
 
        1 If CT is not generic, SOURCE does not conform to CT.

        2 If CT is generic, SOURCE does not conform to the type obtained from
          CT by replacing every formal generic parameter by its generic
          constraint.

        3 cp has exactly one formal argument, of a type ARG.

        4 SOURCE conforms or converts to ARG.

        5 SOURCE’s base type is different from the base type of any other 
          conversion type listed in the Creation_clause of C.

        6 SOURCE is a valid type for conversion
