code: private FieldElement readField(
        Location location, String documentation, @Nullable Field.Label label, String type) {
      String name = reader.readName();
      reader.require('=');
      int tag = reader.readInt();
  
      List<OptionElement> options = new OptionReader(reader).readOptions();
      reader.require(';');
  
      options = new ArrayList<>(options); // Mutable copy for extractDefault.
      String defaultValue = stripDefault(options);
  
      documentation = reader.tryAppendTrailingDocumentation(documentation);
      return new FieldElement(
          location,
          label,
          type,
          name,
          defaultValue,
          tag,
          documentation,
          new ArrayList<>(options));
    }
docs: Reads an field declaration and returns it.