constraints empty_carries/1, request_consistent/1.

  empty_carries([F|Z]) <=>
     ( F=empty(B)         -> B::1..3,
                             not_holds_all(carries(B,_,_),Z) ;
       F=carries(B,R1,R2) -> B::1..3, [R1,R2]::1..6,
                             not_holds(empty(B),Z),
                             not_holds_all(carries(B,_,_),Z) ;
       true ),
     empty_carries(Z).

  request_consistent([F|Z]) <=>
     ( F=request(R1,R2) -> [R1,R2]::1..6 ; true ),
     request_consistent(Z).