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), a subexpression between parenthesis, a sequence between curly braces
or a table literal (such as [[1,2],3,[1,]]). 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.