1 | 8. mem_f(T;u;L) La':T*.
( t:T. u = t mem_f(T;t;v)  mem_f(T;t;L) mem_f(T;t;La'))
& ( t:T. mem_f(T;t;La')  u = t mem_f(T;t;v))
& (||La'|| 1  mem_f(T;hd(La');L)) |
2 | 8. mem_f(T;u;L) La':T*.
( t:T. u = t mem_f(T;t;v)  mem_f(T;t;L) mem_f(T;t;La'))
& ( t:T. mem_f(T;t;La')  u = t mem_f(T;t;v))
& (||La'|| 1  mem_f(T;hd(La');L)) |