def extract$($s: str$,$ start: int, stop: int$)->$ str:

$"""$Return the substring of s from index start $($inclusive$)$ to index

stop $($exclusive$).$

Precondition: $0<=$ start $<=$ stop $<=$ len$($s$)$

$>>>$ extract$(\text{'}$birthday$\text{'},1,4)$

'irt'

$>>>$ extract$(\text{'}$toronto$\text{'},2,5)$

'ron'

$"""$