be added to complete the set of diagrams. The only exception is diagram 44 that is already totally symmetric so that in the final end there are 93 two-loop box diagrams. The generic 47 diagrams are grouped into 6 topology classes. The first 5 topologies contain only a virtual threshold for $Q^2 > 4m_t^2$. The diagrams of topology 6 on the other hand develop a second threshold for $Q^2>0$, because two virtual gluon lines next to the external gluons can be cut. This implies that the form factors are complex in the entire $Q^2$ range. Therefore, a dedicated treatment of this last topology in terms of a suitably constructed infrared subtraction term to isolate the associated infrared singularities is required.

In the following, we will exemplify our method for the boxes 39 of topology 5 and 45 of topology 6. The diagrams of topologies 1–5 are treated analogously to box 39 and those of topology 6 analogously to box 45. The algebraic manipulation of the traces and projections onto the form factors have been performed with the help of the symbolic tools [FORM]{} [@Vermaseren:2000nd; @Kuipers:2012rf], [Reduce]{} [@Hearn:1971zza], and [Mathematica]{} [@Mathematica]. Our method of Feynman parametrization and end-point subtraction to isolate 