A class or interface declaration.

Examples (multi-line):

shared class PrintableWrapper<T>(shared T wrapped)
        satisfies Printable {
    shared actual void print() {
        package.print(wrapped else "<null>");
    }
}

shared interface 3DSized<Unit>
        given Unit satisfies Numeric<Unit> {
    shared formal Unit width;
    shared formal Unit length;
    shared formal Unit height;
}