At:
l before-iff2121
1.
T: Type
2.
L: T List
3.
u: T
4.
v: T List
5.
x,y:T. x before y v (L1,L2,L3:T List. v = (L1 @ [x / (L2 @ [y / L3])]))
6.
x: T
7.
y: T
8.
L2: T List
9.
L3: T List
10.
u = x
11.
v = (L2 @ [y / L3])
x = u & (y v) x before y v
By:
OrLeft
THEN
HypSubst -1 0
THEN
ObviousConcl
Generated subgoals:
x,y:T. x before y 
v 
(
L1,L2,L3:T List. v = (L1 @ [x / (L2 @ [y / L3])]))
x = u & (y 
x before y