def adj_fa(tm, fa_conc, fa_conc_0=None):
    """Adjust secondary structure melting temperature based on formamide
    concentration. Method adapted to work with OligoArrayAux output.
    Based on McConaughy, Biochemistry(8), 1969
    Method based on Wright et al not implemented, as m-value is not available.
    
    Args:
      tm (float): melting temperature, in K.
      fa_conc (float): formamide concentration in %v,v.
      fa_mode (string): formamide correction lavel.
      fa_conc_0 (float): initial formamide concentration in %v,v.
    
    Returns:
      float: corrected melting temperature.
    """
    if type(None) == type(fa_conc_0):
        fa_conc_0 = 0
    dfac = fa_conc - fa_conc_0
    if 0 == dfac:
        return tm
    else:
        return tm - 0.72 * dfac
