@Override
    public NotNode setChildren(List<ExpressionNode> children) {
        if (children.size() != 1) throw new IllegalArgumentException("Expected 1 children but got " + children.size());
        return new NotNode(children.get(0));
    }
A node which flips the logical value produced from the nested expression.

@author lesters
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/NotNode.java