docs: public static Matcher<ExpressionTree> isInstanceField() {
      return new Matcher<ExpressionTree>() {
        @Override
        public boolean matches(ExpressionTree expressionTree, VisitorState state) {
          Symbol symbol = ASTHelpers.getSymbol(expressionTree);
          return symbol != null && symbol.getKind() == ElementKind.FIELD && !symbol.isStatic();
        }
      };
    }