A condition.

There are four kinds of conditions:

• a boolean condition is satisfied when a boolean expression evaluates to true,
• an assignability condition is satisfied when a value satisfies a specified type,
• an existence condition is satisfied when a value is not null, and
• a nonemptiness condition is satisfied when a sequential value is not null and not empty.

Examples:

exists first = string.first
type in { type_ws, type_comment }
is Float number
nonempty members