Each of the language structure elements (concepts, interfaces, expressions, etc.) may have Properties.
Properties can be lists; this is indicated by square brackets after the type name. There are three types of properties.

Primitive properties have as type identifier, string, number, or boolean, and are always contained in the concept. Primitive properties may also be lists.

// Insurance/src/defs/language-main.ast#L25-L26

name: identifier;               // internal name
isUnderConstruction: boolean;   // defines whether this base product is still 'raw'

Parts have as type one of the concepts (including expression concepts and limited concepts) or interfaces in the language, and are contained in the concept that holds the property (as in the UML composition relationship). Parts are also referred to as children. In the example below body and parameters are parts.

// Insurance/src/defs/language-main.ast#L72-L73

body: DocuExpression;               // the actual calculation definition
parameters: Parameter[];            // any parameters

References also have as type one of the concepts or interfaces in the language, but a reference is not contained in its concept (as in the UML association relationship). References are indicated by the keyword reference in front of the definition.

References are always by name, therefore the referred concept must have a name property of type identifier.

In the following example the concept InsuranceProduct holds a list of references to InsuranceParts in the property parts.

// Insurance/src/defs/language-main.ast#L53-L65

concept InsuranceProduct {
    name: identifier;                       // internal name
    productName: string;                    // name by which this product is known to the public
    themes: InsuranceTheme[];               // the 'kind' of insurance
    advertisedPremium: EuroLiteral;         // the premium as known to the public
    nrPremiumDays: PremiumDays;             // the number of days for which the advertised premium is calculated
    reference parts: InsurancePart[];       // optionally, known parts can be included by reference
    reference basedOn: BaseProduct[];       // the BaseProducts from which the parts are taken

    riskAdjustment?: PercentageLiteral;     // an adjustment to the risk of the separate parts, e.g. caused by the combination of the parts
    calculation: DocuExpression;            // the premium as calculated based on the parts
    helpers: CalcFunction[];                // helper functions used to calculate the premium
}

The concept InsurancePart has a property name: identifier.

// Insurance/src/defs/language-main.ast#L44-L45

concept InsurancePart {
    name: identifier;                       // internal name

Properties may be optional. This is indicated using a question mark after the property name. Lists are always considered to be optional, i.e. they may be empty — there is no need for the question mark there.

As of Freon 2.0, primitive properties may also be defined as optional, just like other property types.

// Insurance/src/defs/language-main.ast#L70-L70

description?: Description;          // an optional description