private Item buildTermSearch(OperatorNode<ExpressionOperator> ast) {
        assertHasOperator(ast, ExpressionOperator.CONTAINS);
        String field = getIndex(ast.getArgument(0));
        if (userQuery != null && indexFactsSession.getIndex(field).isAttribute()) {
            userQuery.trace("Field '" + field + "' is an attribute, 'contains' will only match exactly (unless fuzzy is used)", 2);
        }
        return instantiateLeafItem(field, ast.getArgument(1));
    }
Build a "pure" not, without any positive terms.
container-search/src/main/java/com/yahoo/search/yql/YqlParser.java