2. r: Term
3. t': Term
4. r': Term
5. t reps t'
6. r reps r'
SUBX(t; r) reps t'/r'
t,r,t',r':Term. t reps t' & r reps r' 
SUBX(t; r) reps t'/r'
| 1 | 1. t: Term 2. r: Term 3. t': Term 4. r': Term 5. t reps t' 6. r reps r' SUBX(t; r) reps t'/r' | 6 steps |
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