VFAC, section 8.5.21

     Assigner Command Validity rule

     An Assigner_mark appearing in the declaration of a
     query q with n arguments (n >= 0) and listing a
     Feature_name fn, called the assigner command for q,
     is valid if and only if it satisfies the following
     conditions:

        1 fn is the identifier of a command c of the class.

        2 c has n + 1 arguments.

        3 The type of c's first argument is the result
          type of q.

        4 For every i in 1..n, the type of the i+1-st
          argument of c is the type of the i-th argument
          of q.
