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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 _string_substr(self, start, length=None):
 """ Pull substrings out of each string value by position and maximum length. Parameters start : int First character to start splitting, indices starting at 0 (like Python) length : int, optional Maximum length of each substring. If not supplied, splits each string to the end Returns ------- substrings : type of caller """

    op = ops.Substring(self, start, length)
    return op.to_expr()