@Override
    public void resolveLevel(int level) {
        if (level != 0) {
            throw new IllegalArgumentException("Expression '" + this + "' not applicable for " +
                                               GroupingOperation.getLevelDesc(level) + ".");
        }
        super.resolveLevel(level);
    }
This class represents a document value in a {@link GroupingExpression}. As such, the subclasses of this can only be
used as document-level expressions (i.e. level 0, see {@link GroupingExpression#resolveLevel(int)}).

@author Simon Thoresen Hult
container-search/src/main/java/com/yahoo/search/grouping/request/DocumentValue.java