1.4.1.4 Indirection in Modified BNF Syntax

An indirection extension is introduced in order to make this new syntax more readable:

If O is a non-terminal symbol, the right-hand side of its definition is substituted for the entire expression !O. For example, the following BNF is equivalent to the BNF in the previous example:

(x [[!O]] y)

O ::=A | B* | C