@Override
    public String toString(ToStringContext<NAMETYPE> context) {
        return "filter_subspaces(" + argument.toString(context) + ", " + function + ")";
    }
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