index is meant to return the index of the first match it finds in the list from
the beginning, it's most simpless implementation is:
```
for i, v in enumerate(self):
if v == value:
return i
throw ValueError(f"Value '{value}' is not in list")
````
if you want the negative index, just subtract the list's length from the
returned index:
```
res = listy.index(10) - len(listy)
```
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