A reference can be a number, a fraction, a text or a variable reference. A reference can be another reference prefixed by an operator (such as !3), an application (like f(3,4,x+2) ), an @-expression, a tabulation (e.g. t[12]), a subexpression between parenthesis, a sequence between curly braces or a table literal (such as [[1,2],3,[1,[2]]]). The precise definition of these can be found in the rules. Notice that instead of production we sometimes say rule or even EBNF-rule. The latter stands for Extended-Backus-Naur-Formalism. Mr. Backus and Mr. Naur contributed a lot to the popularisation of (non-formal) context free grammars.