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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 which(program):
 """ returns the path to an executable or None if it can't be found """

    def is_exe(_fpath):
        return os.path.isfile(_fpath) and os.access(_fpath, os.X_OK)

    fpath, fname = os.path.split(program)
    if fpath:
        if is_exe(program):
            return program
    else:
        for path in os.environ["PATH"].split(os.pathsep):
            exe_file = os.path.join(path, program)
            if is_exe(exe_file):
                return exe_file
    return None