code: public List<A> findAllAttributesByQualifier(String qualifier) {
          if (StringUtil.isBlank(qualifier) || !attributesPerQualifier.containsKey(qualifier)) return Collections.emptyList();
          return Collections.unmodifiableList(attributesPerQualifier.get(qualifier));
      }
docs: Returns a {@code List} of all attributes that share the same qualifier.<br/>
  Never returns null, but may return an empty list. The returned {@code List} is immutable.
  
  @return a {@code List} of all attributes with the specified qualifier.