Subsections

- Exercises

# Summary

United modes introduce no new values. A united mode can have any mode as one of its constituents except a mode which can be firmly coerced to itself. The uniting coercion is available in firm contexts. Because the values supplied to `print` or `read` are united, the context of the parameter of those procedures is firm. A conformity clause is used to extract the constituent mode or value. The mode `VOID` can be one of the constituents of a united mode and is useful to signal an exceptional yield from a procedure. United modes are used in a variety of ways.

## Exercises

8.9
  Write a declaration for the united mode `CRIB` whose constituent modes are `CHAR`, `REAL`, `INT` and `BOOL`. Ans
8.10
  Write a declaration for the operator `UABS` which has a single operand of mode `CRIB` and which yields the absolute value of its operand. Ans
8.11
  Write four formulæ which use `UABS` and a denotation for each of the constituent modes of `CRIB`. Ans