SUNDeltaContract[exp] substitutes for all SUNDelta in exp SUNDeltaContract, contracts the adjoint \text{SU}(N) indices and resubstitutes SUNDelta. SUNDeltaContract[i, j] is the Kronecker-delta for \text{SU}(N) in the adjoint representation with contraction properties. It wraps the head SUNIndex around its arguments.
SUNDelta[SUNIndex[a], SUNIndex[b]]^2
SUNDeltaContract[%]\left(\delta ^{ab}\right)^2
N^2-1