Tertiaries are formulæ and NIL.
Formulæ were covered in chapter two. All that
needs to be said here is that a formula can consist solely of a single
secondary or primary or enclosed
clause although this is not usual. If a formula, containing at least
one operator, is to be used as a primary or a secondary, it must be
enclosed in parentheses (or BEGIN and END).
For example, in the formula next OF (H declarer), where
H = (INT)REF HAND and HAND = STRUCT(...,REF HAND
next), the formula must be surrounded by parentheses to make it
into a secondary.
The only name having a denotation is NIL. Its mode is
REF whatever. In other words, it can have
any mode which starts with REF. It does not refer to any
value and, although it must only occur in a strong context, it cannot
be coerced. Its uses are described in the next chapter.