SUNFDeltaContract[exp] substitutes for all SUNFDelta in exp SUNFDeltaContract, contracts the fundamental \text{SU}(N) indices and resubstitutes SUNFDelta.SUNFDeltaContract[i, j] is the Kronecker-delta for \text{SU}(N) in the fundamental representation with contraction properties. It wraps the head SUNFIndex around its arguments.
Overview, SUNFDelta, SUNFIndex.
SUNFDelta[SUNFIndex[a], SUNFIndex[b]]^2
SUNFDeltaContract[%]\delta _{ab}^2
N