 
  St:Type, l:St*, f:(St
St:Type, l:St*, f:(St
 St), s:St.
mem_f(St;s;(+f)(l))
St), s:St.
mem_f(St;s;(+f)(l)) 
 mem_f(St;s;l)
 mem_f(St;s;l)  (
 ( s':St. mem_f(St;s';l)  &  f(s') = s)
s':St. mem_f(St;s';l)  &  f(s') = s)| 1 | 1. St: Type 2. l: St* 3. f: St   St 4. s: St 5. mem_f(St;s;(+f)(l))  mem_f(St;s;l)  (  s':St. mem_f(St;s';l)  &  f(s') = s) | 
| 2 | 1. St: Type 2. l: St* 3. f: St   St 4. s: St 5. mem_f(St;s;l)  mem_f(St;s;(+f)(l)) | 
| 3 | 1. St: Type 2. l: St* 3. f: St   St 4. s: St 5. s': St 6. mem_f(St;s';l) 7. f(s') = s  mem_f(St;s;(+f)(l)) | 
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