# Expressions

- Selectors as Expressions

  Every selector is a conditional expression. A selector that evaluates to true matches the message; a selector that evaluates to false or unknown does not match.

- Arithmetic Expression

  Arithmetic expressions are composed of numeric literals, identifiers (that evaluate to numeric literals), arithmetic operations, and smaller arithmetic expressions.

- Conditional Expression

  Conditional expressions are composed of comparison operations, logical operations, and smaller conditional expressions.

- Order of Evaluation

  Order of evaluation is left-to-right, within precedence levels. Parentheses override this order.

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