def _get_low_pressure(self, g_l, d_l, frac_cells):
    """
    Obtains the coefficients of the (projected) lower-dimensional pressures

    Parameters
    ----------
    g_l : PorePy object
        Lower-dimensional grid.
    d_l : Dictionary
        Lower-dimensional data dictionary.
    frac_cells : NumPy nd-Array
        Lower-dimensional fracture cells
        
    Raises
    ------
    ValueError
        If the pressure has not been reconstructed
        
    Returns
    -------
    p_low : NumPy nd-Array
        Coefficients of the (projected) lower-dimensional pressures

    """

    # Retrieve lower-dimensional reconstructed pressure coefficients
    if "recon_p" in d_l[self.estimates_kw]:
        p_low = d_l[self.estimates_kw]["recon_p"].copy()
    else:
        raise ValueError("Pressure must be reconstructed first")
    p_low = p_low[frac_cells]
    
    return p_low