"An optional type, that is, a shortcut for `UnionType(definiteType, nullType)`.
shared class OptionalType(definiteType)
        extends PrimaryType() {
    "The definite type.
     Note that it doesn’t _have_ to be definite; while useless,
     it’s allowed to use another [[OptionalType]] as the definite type."
    shared PrimaryType definiteType;
    shared actual [PrimaryType] children = [definiteType];
    shared actual Result transform<out Result>(Transformer<Result> transformer)
            => transformer.transformOptionalType(this);
    shared actual void visit(Visitor visitor)
            => visitor.visitOptionalType(this);
    shared actual Boolean equals(Object that) {
        if (is OptionalType that) {
            return definiteType == that.definiteType;
        } else {
            return false;
    shared actual Integer hash
            => 31 * definiteType.hash;
    shared OptionalType copy(PrimaryType definiteType = this.definiteType) {
        value ret = OptionalType(definiteType);
        return ret;