opStack Stack of vertically concatenated operators.
opStack(WEIGHTS, OP1, OP2, ...) creates a stacked
operator consisting of the vertical concatenation of all
operators;
[WEIGHT1*OP1
WEIGHT2*OP2
...
WEIGHTn*OPn]
If the same weight is to be applied to each operator, set
WEIGHTS to a scalar. When WEIGHTS is empty [], it is set to
one. The WEIGHT parameter can be omitted as long as OP1 is not
a vector of length (n-1); in which case there is no way to
decide whether it is a weight vector or operator.