  In this paper we present a description of the universal approach for analytic
calculations for a certain class of $J$ functions for six topologies of the
boxes for process $ud\rightarrow WA$. These functions $J$ arise at the
reduction of infrared divergent box diagrams. The standard Passarino--Veltman
reduction of four-point box diagram with an internal photon line connecting two
external lines on the mass shell leads to infrared-divergent and mass-singular
$D_0$ functions. In the system SANC a systematic procedure is adopted to
separate both types of singularities into the simplest objects, namely $C_0$
functions. The functions $J$, in turn, are represented as certain linear
combinations of the standard $D_0$ and $C_0$ functions. The subtracted $J$
functions are free of both types of singularities and are expressed as explicit
and compact linear combinations of dilogarithm functions. We present extensive
comparisons of numerical results of SANC with those obtained with the aid of
the LoopTools package.
