simp [v.startPos]
case a
l m : List Char
c : Char
r : List Char
s : Substring
v : ValidFor l m (c :: r) s
this✝ : s.stopPos < s.str.endPos
this : { data := l ++ (m ++ c :: r) }.get { byteIdx := String.utf8Len l + String.utf8Len m } = c
⊢ { str := { data := l ++ (m ++ c :: r) }, startPos := s.startPos,
          stopPos := { byteIdx := String.utf8Len l + String.utf8Len m } + c }.startPos.byteIdx =
    String.utf8Len l
no goals