class ResolvedUnionType implements ResolvedType {

    private List<ResolvedType> elements;

    public ResolvedUnionType(List<ResolvedType> elements) {
        if (elements.size() < 2) {
            throw new IllegalArgumentException("An union type should have at least two elements. This has " + elements.size());
        }
        this.elements = new LinkedList<>(elements);
    }

    public Optional<ResolvedReferenceType> getCommonAncestor() {<FILL_FUNCTION_BODY>}

    @Override
    public boolean equals(Object o) {
        if (this == o)
            return true;
        if (o == null || getClass() != o.getClass())
            return false;
        ResolvedUnionType that = (ResolvedUnionType) o;
        return new HashSet<>(elements).equals(new HashSet<>(that.elements));
    }

    @Override
    public int hashCode() {
        return new HashSet<>(elements).hashCode();
    }

    @Override
    public String describe() {
        return String.join(" | ", elements.stream().map(ResolvedType::describe).collect(Collectors.toList()));
    }

    @Override
    public boolean isAssignableBy(ResolvedType other) {
        return elements.stream().allMatch(e -> e.isAssignableBy(other));
    }

    @Override
    public boolean isUnionType() {
        return true;
    }

    @Override
    public ResolvedUnionType asUnionType() {
        return this;
    }

    /*
     * Returns the list of the resolved types
     */
    public List<ResolvedType> getElements() {
        return elements;
    }
}

        Optional<List<ResolvedReferenceType>> reduce = elements.stream()
        		.map(ResolvedType::asReferenceType)
        		.map(rt -> rt. getAllAncestors(ResolvedReferenceTypeDeclaration.breadthFirstFunc))
        		.reduce((a, b) -> {
            ArrayList<ResolvedReferenceType> common = new ArrayList<>(a);
            common.retainAll(b);
            return common;
        });
        return reduce.orElse(new ArrayList<>()).stream().findFirst();
    
<no_super_class>