code: void compareFixedTypes(Schema oldSchema,
                                                  Schema newSchema,
                                                  List<Message> messages,
                                                  String name) {
          if(oldSchema == null || newSchema == null || oldSchema.getType() != Schema.Type.FIXED) {
              throw new IllegalArgumentException("Old schema must be FIXED type. Name=" + name
                                                 + ". Type=" + oldSchema);
          }
  
          if(newSchema.getType() != Schema.Type.FIXED) {
              messages.add(new Message(Level.ERROR, "Illegal type change from " + oldSchema.getType()
                                                    + " to " + newSchema.getType() + " for field "
                                                    + name));
              return;
          }
  
          if(newSchema.getFixedSize() != oldSchema.getFixedSize()) {
              messages.add(new Message(Level.ERROR, "Illegal size change for fixed type field "
                                                    + name));
          }
      }
docs: /* package private