circuit top :
   module subtracter :
      input x:UInt
      input y:UInt
      output z:UInt
      z := sub-mod(x, y)

   module gcd :
      input a: UInt(16)
      input b: UInt(16)
      input e: UInt(1)
      output z: UInt(16)
      output v: UInt(1)

      reg x: UInt
      reg y: UInt
      x.init := UInt(0)
      y.init := UInt(42)

      when greater(x, y) :
         inst s of subtracter
         s.x := x
         s.y := y
         x := s.z
      else :
         inst s2 of subtracter
         s2.x := x
         s2.y := y
         y := s2.z
      
      when e :
         x := a
         y := b

      v := equal(v, UInt(0))
      z := x

   module top :   
      input a: UInt(16)
      input b: UInt(16)
      output z: UInt

      inst i of gcd
      i.a := a
      i.b := b
      i.e := UInt(1)
      z := i.z