S9fES  (require-extension <extension> ...)  ==>  unspecific

When the given <extension>s have been compiled into the interpreter
running this form, the form evaluates to an unspecific value. When
at least one of the given <extension>s is absent, an error is
signalled.

Each extension may be either a symbol or a list of the form

(or <name> ...)

where each <name> names an extension. For the REQUIRE-EXTENSIONS form
to succeed, it is sufficient if one of the extensions named in (or ...)
is present. For example:

(require-extension curses)

requires the CURSES extension to be compiled-in, and

(require-extension (or sys-unix sys-plan9))

requires either the SYS-UNIX or the SYS-PLAN9 extension to be present.
(In the unlikely case that both the SYS-UNIX and SYS-PLAN9 extensions
would be detected, the form would still succeed).
