“…Gianni et al, [29], give a succinct description of this construction and of a more general one which allows us to compute a basis for the kernel of a given ring-homomorphism from P[t/i,... ,y p ] to P[xi,..., x n ]/I which is invariant on R . The paper by Shannon and Sweedler, [34], is more detailed and describes a subalgebra membership test as well.…”