VFAV, section 8.5.25 

     Alias validity

     An Alias clause is alias-valid for a feature f of a
     class C if and only if it satisfies the following
     conditions:

        1 If it lists an Operator op: f is a query; no
          other query of C has an Operator alias using the
          same operator and the same number of arguments;
          and either: op is a Unary and f has no argument,
          or op is a Binary and f has one argument.

        2 If it lists a Bracket alias: f is a query with at
          least one argument, and no other feature of C has
          a Bracket alias.

        3 If it includes a convert mark: it lists an
          Operator and f has one argument.

        4 If it lists one of the "semistrict" operators
          "and then", "or else" and "implies": C is the
          Kernel Library class BOOLEAN.