code: public static @CheckForNull
      TypeQualifierAnnotation combineReturnTypeAnnotations(TypeQualifierAnnotation a, TypeQualifierAnnotation b) {
          return combineAnnotations(a, b, combineReturnValueMatrix);
      }
docs: Combine return type annotations.
  
  @param a
  a TypeQualifierAnnotation used on a return value
  @param b
  another TypeQualifierAnnotation used on a return value
  @return combined return type annotation that is at least as narrow as
  both <code>a</code> or <code>b</code>, or null if no such
  TypeQualifierAnnotation exists