@Override
    public PrimitiveTensorFunction<NAMETYPE> toPrimitive() {
        return new MapSubspaces<>(argument.toPrimitive(), function.toPrimitive());
    }
The <i>map_subspaces</i> tensor function transforms each dense subspace in a (mixed) tensor

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