@Override
    public PrimitiveTensorFunction<NAMETYPE> toPrimitive() {
        return new FilterSubspaces<>(argument.toPrimitive(), function.toPrimitive());
    }
The <i>filter_subspaces</i> tensor function selects some cells or subspaces in a mapped or mixed tensor

@author arnej
vespajlib/src/main/java/com/yahoo/tensor/functions/FilterSubspaces.java