This chapter (and the rest of the documentation) uses a slightly different notation than the character-based grammars of the Racket Essentials chapter. The grammar for a use of a syntactic form something is shown like this:
(something [id ...+] an-expr ...)
The italicized meta-variables in this specification, such as id and an-expr, use the syntax of Racket identifiers, so an-expr is one meta-variable. A naming convention implicitly defines the meaning of many meta-variables:
A meta-variable that ends in id stands for an identifier, such as x or my-favorite-martian.
A meta-identifier that ends in keyword stands for a keyword, such as #:tag.
A meta-identifier that ends with expr stands for any sub-form, and it will be parsed as an expression.
A meta-identifier that ends with body stands for any sub-form; it will be parsed as either a local definition or an expression. The last body must be an expression; see also Internal Definitions.
Square brackets in the grammar indicate a parenthesized sequence of forms, where square brackets are normally used (by convention). That is, square brackets do not mean optional parts of the syntactic form.
A ... indicates zero or more repetitions of the preceding form, and ...+ indicates one or more repetitions of the preceding datum. Otherwise, non-italicized identifiers stand for themselves.
Based on the above grammar, then, here are a few conforming uses of something:
(something [x]) (something [x] (+ 1 2)) (something [x my-favorite-martian x] (+ 1 2) #f)
Some syntactic-form specifications refer to meta-variables that are not implicitly defined and not previously defined. Such meta-variables are defined after the main form, using a BNF-like format for alternatives:
(something-else [thing ...+] an-expr ...)
thing = thing-id | thing-keyword
The above example says that, within a something-else form, a thing is either an identifier or a keyword.