# Tertiaries

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.