def get_marginal_rna_obs(r: int, p_max: int):
    """Get the observable for a marginalized RNA abundance."""
    marginal = ''
    for p in range(p_max-1):
        marginal += f'x_{r}_{p} + '
    marginal += f'x_{r}_{p_max-1}'

    return {'name': f'x_r{r}', 'formula': marginal}