docs: public static Matcher<ExpressionTree> isVariable() {
      return new Matcher<ExpressionTree>() {
        @Override
        public boolean matches(ExpressionTree expressionTree, VisitorState state) {
          Symbol symbol = ASTHelpers.getSymbol(expressionTree);
          if (symbol == null) {
            return false;
          }
          return symbol.getKind() == ElementKind.LOCAL_VARIABLE
              || symbol.getKind() == ElementKind.PARAMETER;
        }
      };
    }