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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def str_slice(arr, start=None, stop=None, step=None):
 """ Slice substrings from each element in the Series or Index. Parameters start : int, optional Start position for slice operation. stop : int, optional Stop position for slice operation. step : int, optional Step size for slice operation. Returns ------- Series or Index of object Series or Index from sliced substring from original string object. See Also -------- Series.str.slice_replace : Replace a slice with a string. Series.str.get : Return element at position. Equivalent to `Series.str.slice(start=i, stop=i+1)` with `i` being the position. Examples -------- 0 koala 1 fox 2 chameleon dtype: object 0 oala 1 ox 2 hameleon dtype: object 0 ko 1 fo 2 ch dtype: object 0 kaa 1 fx 2 caeen dtype: object 0 kl 1 f 2 cm dtype: object Equivalent behaviour to: 0 kl 1 f 2 cm dtype: object """

    obj = slice(start, stop, step)
    f = lambda x: x[obj]
    return _na_map(f, arr)