Get the previous n data from file.

        Keyword argument:
            n -- number of structs to be retrieved (default 1)
                 Must be greater than 0.

        Return:
            A data in the format of obj_fmt, if n = 1. A list of
            structs, otherwise.

def prev(self, n=1):
        """Get the previous n data from file.

        Keyword argument:
            n -- number of structs to be retrieved (default 1)
                 Must be greater than 0.

        Return:
            A data in the format of obj_fmt, if n = 1. A list of
            structs, otherwise.
        """
        # Current position - #data
        i = abs(self.tell - n)

        # Get the next n data starting from i
        return self.get(i, n)