def _unparse_side(unside):
    """Return string representation of a left/right hand side of a reaction.

    Parameters
    ----------
    unside : sequence of tuple

    Returns
    -------
    side : str

    Examples
    --------
    >>> _unparse_side([(1, 'A'),
    ...     (1, 'B'),
    ...     (2, 'C'),
    ...     (500, 'D')])
    'A + B + 2 C + 500 D'

    """
    return " + ".join(
        f"{compound}" if coefficient == 1 else f"{coefficient} {compound}"
        for coefficient, compound in unside
    )