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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 pos(string, substr, start):
 """ Find the first occurrence in a string of a substring, starting at a specified location, searching forward. http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/pos_c.html :param string: Any character string. :type string: str :param substr: Substring to locate in the character string. :type substr: str :param start: Position to begin looking for substr in string. :type start: int :return: The index of the first occurrence of substr in string at or following index start. :rtype: int """

    string = stypes.stringToCharP(string)
    substr = stypes.stringToCharP(substr)
    start = ctypes.c_int(start)
    return libspice.pos_c(string, substr, start)