I wanted to define a 4th order tensor in the following way,

    restart;
    with(Physics);
    Setup(mathematicalnotation = true);
    delta := KroneckerDelta;
    L[i, j, k, l] := lambda*delta[i, j]*delta[k, l]+mu*(delta[i, k]*delta[j, l]+delta[i, l]*delta[j, k]);

but when I type L[1, 1, 1, 1] it doesn't replace i, j, j and l by 1. i
do i need to define it in a different way?