docs: public static Matcher<VariableTree> isField() {
      return new Matcher<VariableTree>() {
        @Override
        public boolean matches(VariableTree variableTree, VisitorState state) {
          Element element = ASTHelpers.getSymbol(variableTree);
          return element.getKind() == ElementKind.FIELD;
        }
      };
    }