Editing ALGOL 68 (section)

===Miscellaneous===
For its technical intricacies, ALGOL 68 needs a cornucopia of methods to deny the existence of something:

 '''SKIP''', "~" or "?"<sup>C</sup> – an undefined value always syntactically valid,
 '''EMPTY''' – the only value admissible to '''VOID''', needed for selecting '''VOID''' in a '''UNION''',
 '''VOID''' – syntactically like a '''MODE''', but not one,
 '''NIL''' or "○" – a name not denoting anything, of an unspecified reference mode,
 () or specifically [1:0]'''INT''' – a ''vacuum'' is an empty array (here specifically of '''MODE''' []'''INT''').
 ''undefined'' – a standards reports procedure raising an exception in the runtime system.
 ℵ – Used in the standards report to inhibit [[type introspection|introspection]] of certain types. e.g. '''SEMA'''

The term '''NIL''' '''IS''' ''var'' always evaluates to '''TRUE''' for any variable (but see above for correct use of '''IS''' :/=:), whereas it is not known to which value a comparison ''x'' < '''SKIP''' evaluates for any integer ''x''.

ALGOL 68 leaves intentionally undefined what happens in case of [[integer overflow]], the integer bit representation, and the degree of numerical accuracy for floating point.

Both official reports included some advanced features that were not part of the standard language.  These were indicated with an ℵ and considered effectively private.  Examples include "≮" and "≯" for templates, the  '''OUTTYPE'''/'''INTYPE''' for crude [[duck typing]], and the '''STRAIGHTOUT''' and '''STRAIGHTIN''' operators for "straightening" nested arrays and structures