simp [h] at this
l r : List Char
it : Iterator
v : ValidFor l r it
h : ¬it.atEnd = true
this : it.atEnd = true ↔ r = []
⊢ ∃ r', r = it.curr :: r'
l r : List Char
it : Iterator
v : ValidFor l r it
h : ¬it.atEnd = true
this : ¬r = []
⊢ ∃ r', r = it.curr :: r'