VTMC, ECMA Eiffel, chapter 8.12.22

    Multiple Constraints rule

    A feature  of name fname  is applicable  in a class C to a
    target x whose type is a Formal_generic_name G constrained
    by two or more types CONST1, CONST2,..., if and only if it
    satisfies the following conditions:


        1 At  least one of the CONST_i has a feature available
          to C whose  generically  constrained name for G in C
          is fname.

        2 If this is the case for  two or more of the CONST_i,
          all the corresponding features are the same.


