# Phrases

A phrase is a declaration or a unit. Declarations yield no value, even if they include an initial assignment. Units are the parts of the language which actually manipulate values. There are 22 different kinds of unit which can be subdivided into 5 classes arranged in a hierarchy:

       Quaternaries
          Tertiaries
             Secondaries
                Primaries
                   Enclosed clauses

where each class includes the lower class. For example, all enclosed clauses are primaries, but not all primaries are enclosed clauses.

The distinctions between different classes of units prevent the writing of ambiguous programs and help to provide the meaning you might expect.

The units in each class are as follows:

- Quaternaries
  - assignments
  - identity relations
  - routine denotations
  - `SKIP`
- Tertiaries
  - formulæ
  - `NIL`
- Secondaries
  - generators
  - selections
- Primaries
  - applied-identifiers
  - calls
  - casts
  - denotations (except routine denotations)
  - slices
- Enclosed clauses
  - case clauses
  - closed clauses
  - collateral clauses
  - conditional clauses
  - conformity clauses
  - loop clauses
  - parallel clauses
  - row-displays
  - structure-displays