I would like to create "square-curly bracket" delimiters, somehow looking as in [this question](https://tex.stackexchange.com/questions/438487/customizing-square-brackets). I came up with the code below, which produces some good results, but has also some inconvenients. For instance we get these examples:

![](https://i.stack.imgur.com/IgVyD.png)
![](https://i.stack.imgur.com/DBLuL.png)
You can see that the fraction bar is not really centered (because of the exponent 'j'), and the formatting under the summation symbols is too big.

My question is:

> 
> How would you proceed to have nice-looking "square-curly" brackets?
> 
> 
> 

---

MWE:

```
\documentclass{article}

\usepackage{tikz, mathtools, titletoc}

\DeclareRobustCommand{\fp}[1]{%
    \let\mybox\relax%
    \newsavebox\mybox%
    \sbox{\mybox}{$#1$}%
    \def\WIDTH{\the\dimexpr\wd\mybox + 6pt \relax}%
    \def\HEIGHT{\the\dimexpr\ht\mybox * 10 / 20 +  \dp\mybox * 10/20 + 2pt\relax}%
    \def\SHIFT{2pt}%
    \begin{tikzpicture}[baseline = -0.57ex]
    \draw  [line width=0.6]  (\SHIFT, \HEIGHT) -- (0, \HEIGHT) -- (0, 1.0pt) -- (-1.5pt, 0) -- (0, -1.0pt) -- (0, -\HEIGHT) -- (\SHIFT, -\HEIGHT);%
    \node[anchor = west] at (-0.01, 0) {\copy\mybox};%
    \draw  [line width=0.6]  (-\SHIFT + \WIDTH, \HEIGHT) -- (0 + \WIDTH, \HEIGHT) -- (0 + \WIDTH, 1.0pt) -- (1.5pt + \WIDTH, 0) -- (0 + \WIDTH, -1.0pt) -- (0 + \WIDTH, -\HEIGHT) -- (-\SHIFT + \WIDTH, -\HEIGHT);%
    \end{tikzpicture}%
}

\begin{document}

$\fp{x}         \qquad          \fp{ \frac{x a}{d} } = \left\{ \frac{x a}{d} \right\}    \leq \frac{1}{2}$

\[      \fp{ -x } = \fp{ \dfrac{a x}{d} } = \fp{ \frac{x}{2} }   = 
\fp{ \dfrac{- t \cdot a^j }{ R } }      =       \fp{ \dfrac{- t  a_j }{ R } }   \]

\[      \sum_{ \fp{r/m} \in S } r       =       \sum_{ \fp{ \frac{r}{m} } \in S } r.        \]

\end{document}

```