Sudoku(4x4) solver in Alloy

gistfile1.txt

sig Digit {}
one sig One, Two, Three, Four extends Digit {}

sig Cell {content: one One + Two + Three + Four}

sig Group {
  cells: set Cell
} {
  no disj c, c': cells | c.content = c'.content
}

sig Row, Column, Matrix extends Group {}

one sig
Ca0, Ca1, Ca2, Ca3,
Cb0, Cb1, Cb2, Cb3,
Cc0, Cc1, Cc2, Cc3,
Cd0, Cd1, Cd2, Cd3 extends Cell {}

one sig Ra, Rb, Rc, Rd extends Row {}
one sig C0, C1, C2, C3 extends Column {}
one sig Ma0, Ma2, Mc0, Mc2 extends Matrix {}

fact {
  Ra.cells = Ca0 + Ca1 + Ca2 + Ca3
  Rb.cells = Cb0 + Cb1 + Cb2 + Cb3
  Rc.cells = Cc0 + Cc1 + Cc2 + Cc3
  Rd.cells = Cd0 + Cd1 + Cd2 + Cd3

  C0.cells = Ca0 + Cb0 + Cc0 + Cd0
  C1.cells = Ca1 + Cb1 + Cc1 + Cd1
  C2.cells = Ca2 + Cb2 + Cc2 + Cd2
  C3.cells = Ca3 + Cb3 + Cc3 + Cd3

  Ma0.cells = Ca0 + Ca1 + Cb0 + Cb1
  Ma2.cells = Ca2 + Ca3 + Cb2 + Cb3
  Mc0.cells = Cc0 + Cc1 + Cd0 + Cd1
  Mc2.cells = Cc2 + Cc3 + Cd2 + Cd3
}

// 1 2 * *
// 3 4 1 *
// * 1 * *
// * * * 1
fact {
  Ca0.content = One
  Ca1.content = Two
  Cb0.content = Three
  Cb1.content = Four
  Cb2.content = One
  Cc1.content = One
  Cd3.content = One
}
 
pred show {}
run show for 20 but 16 Cell