Panel a: possible evolutions of C_H II with redshift. ML models are shown for the two models of Sect. 2.2: blue for the first, coral for the second. Dotted horizontal lines correspond to the mean value of C_H II (z) for z > 6.8, where outputs of the model are used in calculations, for the model of the corresponding colour. The vertical line is located at z = 6.8. Lines of lighter colours represent various outputs of the sampling of the corresponding model. Panel b: redshift evolution of Q_H II for the same models of C_H II(z). Inferences are compared to a result with C_H II(z) = 3 in purple dashed line, and to the theoretical models of Planck Collaboration Int. XLVII (2016): a redshift-symmetric model in black and a redshift-asymmetric model in brown. References. HM12: Haardt & Madau (2012) or Eq. (10). I07: Iliev et al. (2007) or Eq. (11).