VAR

  a[2]
  b[2]

DEFINE

  b[0] := 
    case
      !(!a[1] & a[0]) & !(a[1] & a[0]) : 1;
      1 : 0;
    esac;
  b[1] := 
    case
      !(!a[1] & !a[0]) & !(a[1] & a[0]) : 1;
      1 : 0;
    esac;

SPEC

  ((a[1] <-> !a[0]) <-> b[1]) & (!a[0] <-> b[0])
