public static Annotations combine(Annotations a1, Annotations a2) {
    Annotations result = new Annotations();

    result.addAll(a1);
    result.addAll(a2);
    result.setImmutable();

    return result;
  }
Constructs an immutable instance which is the combination of the two given instances. The two
instances must contain disjoint sets of types.

@param a1 {@code non-null;} an instance
@param a2 {@code non-null;} the other instance
@return {@code non-null;} the combination
@throws IllegalArgumentException thrown if there is a duplicate type
android/android_framework/dex-maker/src/main/java/com/android/dx/rop/annotation/Annotations.java