f(t))
t:Term. s(t) 
Termt; SUBX(X; UP(X)))t:Term. f(t) Termt,r:Term. SUBX(t; r) Term| Who Cites s? | |
| s | Def s(t) == SUBX(f(t); f(t)) |
| Thm* t:Term. s(t) Term | |
| f | Def f(t) == SUBX(t; SUBX(X; UP(X))) |
| Thm* t:Term. f(t) Term | |
| subx | Def SUBX(t; e) == t[e/X] |
| Thm* t,r:Term. SUBX(t; r) Term |
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