sudoku(L):-

 
L=   [
    A1,A2,A3,A4,
    B1,B2,B3,B4,
    C1,C2,C3,C4,
    D1,D2,D3,D4
    
   ],

   fd_domain(L,1,4),
   fd_all_different([A1,A2,A3,A4]),
   fd_all_different([B1,B2,B3,B4]),
   fd_all_different([C1,C2,C3,C4]),
   fd_all_different([D1,D2,D3,D4]),
   fd_all_different([A1,B1,C1,D1]),
   fd_all_different([A2,B2,C2,D2]),
   fd_all_different([A3,B3,C3,D3]),
   fd_all_different([A4,B4,C4,D4]),
   fd_all_different([A1,A2,B1,B2]),
   fd_all_different([A3,A4,B3,B4]),
   fd_all_different([C1,C2,D1,D2]),
   fd_all_different([C3,C4,D3,D4]),
   
   A2#=2,
   B1#=1,
   C4#=4,
   D3#=1,
   
   fd_labeling(L).