GSLN[p,n,nb] denotes the negative component in the lightcone decomposition of the slashed Dirac matrix (\gamma \cdot p) along the vectors n and nb. It corresponds to \frac{1}{2} (n \cdot p) (\gamma \cdot \bar{n}).
If one omits n and nb, the program will use default vectors specified via $FCDefaultLightconeVectorN and $FCDefaultLightconeVectorNB.
Overview, DiracGamma, GALP, GALN, GALR, GSLP, GSLR.
GSLN[p, n, nb]\frac{1}{2} \left(\overline{n}\cdot \overline{p}\right) \bar{\gamma }\cdot \overline{\text{nb}}
StandardForm[GSLN[p, n, nb] // FCI]\frac{1}{2} \;\text{DiracGamma}[\text{Momentum}[\text{nb}]] \;\text{Pair}[\text{Momentum}[n],\text{Momentum}[p]]
Notice that the properties of n and nb vectors have to be set by hand before doing the actual computation
GSLN[p, n, nb] . GSLN[q, n, nb] // DiracSimplify\frac{1}{4} \overline{\text{nb}}^2 \left(\overline{n}\cdot \overline{p}\right) \left(\overline{n}\cdot \overline{q}\right)
FCClearScalarProducts[]
SP[n] = 0;
SP[nb] = 0;
SP[n, nb] = 2;GSLN[p, n, nb] . GSLN[q, n, nb] // DiracSimplify0
FCClearScalarProducts[]