KDD[i, j]
is the Kronecker delta in dimensions.
[i, j] KDD
[KDD[i, j] KDD[i, j]] Contract
[a, b] // StandardForm
KDD
(*KDD[a, b]*)
[KDD[a, b]] // StandardForm
FCI
(*CartesianPair[CartesianIndex[a, -1 + D], CartesianIndex[b, -1 + D]]*)
[FCI[KDD[a, b]]] // StandardForm
FCE
(*KDD[a, b]*)