SUBROUTINE QISPLT(WEIGHT) - Weight for photon splitting into quark-antiquark pair

This subroutine calculates the weight (WEIGHT)

$$W_{q^\prime} = \frac{1}{x^\prime z}\, P_{q^\prime}^{{ (I)}},$$ (1)

where the factor $P_{q^\prime}^{{ (I)}}$ accounts for the flux of virtual (anti-)quarks $q^\prime$ in the $I$-background entering the $I$-induced $q^\prime p$-subprocess from the photon side [1,2],

$$P_{q^\prime}^{{ (I)}} = \frac{3}{16\,\pi^3}\,\frac{x_{\rm Bj}}{x^... ...} -\frac{1}{x^{\prime}}-\frac{Q^{\prime 2}}{S x_{\rm Bj}y_{\rm Bj}}\right) \, .$$ (2)