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An expression is a rule for calculating a value based on the conventional rules of algebra for left-to-right evaluation of operators and operator precedence. Expressions are composed of factors, terms, and simple expressions. 

Factors are evaluated first and consist of individual constants or variables or function designators or array-parameter bounds or set constructors (see Chapter 8). A factor may also consist of the operator not applied to another factor representing a Boolean value. A factor may also comprise an expression enclosed within parentheses which is evaluated independently of preceding and following operators. 

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