private Item instantiateLeafItem(String field, OperatorNode<ExpressionOperator> ast) {
        return switch (ast.getOperator()) {
            case LITERAL, VARREF -> instantiateWordItem(field, ast, null);
            case CALL -> instantiateCompositeLeaf(field, ast);
            default -> throw newUnexpectedArgumentException(ast.getOperator().name(),
                                                            ExpressionOperator.CALL, ExpressionOperator.LITERAL);
        };
    }
Build a "pure" not, without any positive terms.
container-search/src/main/java/com/yahoo/search/yql/YqlParser.java