V = SEQUENTIAL(W1,W2)
The two mappings W1 and W2 are combined into a single mapping V. Note that SEQUENTIAL(W1,W2) is equivalent to W1*W2. If W2 is a mapping of a type 'combiner', it is called to make a combination attempt. SEQUENTIAL(A,W) maps the dataset A by the sequential mapping W.
This routine is automatically called to execute W = W1*W2 or B = A*W2 in case W2 is a sequential mapping. It should not be directly called by users.