def index(self, value, start=0):
    if self._isEmpty():
        raise ValueError('OurList.index(x): x not in list')
    elif start == 0 and self._head == value:
        return 0
    else:
        return 1 + self._rest.index(value, max(start-1,0))

def search(lexicon, target):
    return _search(lexicon, target, 0, len(target))

def _search(lexicon, target, start, stop):
    if start >= stop:
        return stop       # if we didn't find item, must go between stop-1 and stop
    else:
        mid = (start+stop)//2
        if target == lexicon[mid]:
            return mid    # that's where we found it
        elif target < lexicon[mid]:
            return _search(lexicon, target, start, mid)
        else:
            return _search(lexicon, target, mid+1, stop)