<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tostype(self, stype):
 """Return a copy of the array with chosen storage type. Returns ------- NDArray or RowSparseNDArray A copy of the array with the chosen storage stype """

        # pylint: disable= no-member, protected-access
        if stype == 'csr':
            raise ValueError("cast_storage from row_sparse to csr is not supported")
        return op.cast_storage(self, stype=stype)