FCLoopBasisExtract[int, {q1, q2, ...}] is an auxiliary
function that extracts the scalar products that form the basis of the
loop integral in int. It needs to know the loop momenta on which the
integral depends and the dimensions of the momenta that may occur in the
integral.
SPD[q, p] SFAD[q, q - p, q - p]
FCLoopBasisExtract[%, {q}, SetDimensions -> {4, D}]\frac{p\cdot q}{(q^2+i \eta ).((q-p)^2+i \eta )^2}
\left\{\left\{p\cdot q,q^2,-2 (p\cdot q)+p^2+q^2\right\},\left\{p\cdot q,q^2\right\},\{-1,1,2\},\left\{p\cdot q,\frac{1}{(q^2+i \eta )},\frac{1}{((q-p)^2+i \eta )}\right\}\right\}
SFAD[p1]
FCLoopBasisExtract[%, {p1, p2, p3}, FCTopology -> True, FCE -> True]\frac{1}{(\text{p1}^2+i \eta )}
\left\{\left\{\text{p1}^2\right\},\left\{\text{p1}^2,\text{p1}\cdot \;\text{p2},\text{p1}\cdot \;\text{p3},\text{p2}^2,\text{p2}\cdot \;\text{p3},\text{p3}^2\right\},\{1\},\left\{\frac{1}{(\text{p1}^2+i \eta )}\right\}\right\}