The datum syntax describes the syntax of syntactic data in terms of a
sequence of lexemes, as defined in the lexical syntax.
The following grammar describes the syntax of syntactic data in terms of
various kinds of lexemes defined in the grammar in section “Lexical
Syntax”:
The lexical syntax #n=datum reads the
same as datum, but also results in datum being
labelled by n, which must a sequence of digits.
The lexical syntax #n# serves as a reference to some
object labelled by #n=; the result is the same object
(in the sense of eq?) as the #n=.
Together, these syntaxes permit the notation of structures
with shared or circular substructure.
(let ((x (list 'a 'b 'c)))
(set-cdr! (cddr x) x)
x) ⇒ #0=(a b c . #0#)
The scope of a datum label is the portion of the outermost
datum in which it appears that is to the right of the label.
Consequently, a reference #n# can occur
only after a label #n=;
it is an error to attempt a forward reference.
In addition, it is an error if the reference appears as the labelled
object itself (as in #n=#n#), because the object
labelled by #n= is not well defined in this case.
