code: private ExtendElement readExtend(Location location, String documentation) {
      String name = reader.readName();
  
      reader.require('{');
      ImmutableList.Builder<FieldElement> fields = ImmutableList.builder();
      while (true) {
        String nestedDocumentation = reader.readDocumentation();
        if (reader.peekChar('}')) break;
  
        Object declared = readDeclaration(nestedDocumentation, Context.EXTEND);
        if (declared instanceof FieldElement) {
          fields.add((FieldElement) declared);
        }
      }
      return new ExtendElement(location, name, documentation, fields.build());
    }
docs: Reads an extend declaration.