DOCBOOK-APPS: Interpreting XML Standard's "productions"

[Zenaan Harkness <zen@getsystems.com>]

The XML standard (as per w3c website), has "productions" such as the
following:

[20]    CData    ::=    (Char* - (Char* ']]>' Char*))

and I would like to understand - or if someone knows a reference that
describes - how these "productions" are defined. Mostly they kind of
make sense - some are even obvious. But as an example, the one above I
know means "CData can contain any characters except for the sequence
']]>' (not counting single quotes)". But why does the above production
mean this?

TIA
Zen

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