∀[T:Type]. ∀L1,L2:T List. L1 ⊆ tl(L2)
⇒ L1 ⊆ L2 supposing ¬↑null(L2)
{ (InductionOnList THEN Auto) }
1. [T] : Type
2. u : T
3. v : T List
4. ∀L2:T List. v ⊆ tl(L2)
⇒ v ⊆ L2 supposing ¬↑null(L2)
5. L2 : T List
6. ¬↑null(L2)
7. [u / v] ⊆ tl(L2)
⊢ [u / v] ⊆ L2