| 1 | 2. x2: Var y:Formula. Dec( x2 = y) |
| 2 | 2. x1: Formula 3. y:Formula. Dec(x1 = y) y@0:Formula. Dec(  x1 = y@0) |
| 3 | 2. x2: Formula 3. x3: Formula 4. y:Formula. Dec(x2 = y) 5. y:Formula. Dec(x3 = y) y@0:Formula. Dec(x2  x3 = y@0) |
| 4 | 2. x2: Formula 3. x3: Formula 4. y:Formula. Dec(x2 = y) 5. y:Formula. Dec(x3 = y) y@0:Formula. Dec(x2  x3 = y@0) |
| 5 | 2. y1: Formula 3. y2: Formula 4. y:Formula. Dec(y1 = y) 5. y:Formula. Dec(y2 = y) y@0:Formula. Dec(y1   y2 = y@0) |