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