| 1: | app(app(app(rec,f),x),0) | → x | |
| 2: | app(app(app(rec,f),x),app(s,y)) | → app(app(f,app(s,y)),app(app(app(rec,f),x),y)) | |
| 3: | APP(app(app(rec,f),x),app(s,y)) | → APP(app(f,app(s,y)),app(app(app(rec,f),x),y)) | |
| 4: | APP(app(app(rec,f),x),app(s,y)) | → APP(f,app(s,y)) | |
| 5: | APP(app(app(rec,f),x),app(s,y)) | → APP(app(app(rec,f),x),y) | |