As mentioned in the comments it is not recommended, and such a solution will make the code much less readable. 

That being said, it can be done. This use a trick suggested by Sebastien Gouezel
cited from the mathtools manual

\documentclass[a4paper]{memoir}
\usepackage{amsmath}

\newcommand\MTkillspecial[1]{% helper macro
  \bgroup
  \catcode`\&amp;=9
  \let\\\relax%
  \scantokens{#1}%
  \egroup
}

\newcommand\paren[1]{
  \left(\vphantom{\MTkillspecial{#1}}\kern-\nulldelimiterspace\right.
  #1
  \left.\kern-\nulldelimiterspace\vphantom{\MTkillspecial{#1}}\right)
}


\begin{document}

\begin{align*}
  A  = {} &amp; \paren{ \frac12 \\ &amp; +3 } 
\end{align*}


\end{document}


I would still manually scale fences in such a situation, as in

\begin{align*}
  A = {} &amp; \biggl( \frac12
  \\
  &amp; +3 \biggr)
\end{align*}


which in my opinion is a lot more readable.

user-defined parentheses command breaking across lines
... apart from western countries like the Australia or the USA themselves.

During my visit to Taiwan, I wasn't able to find the dishes I'd typically encounter in a Chinese restaurant in Australia or the United States, such as sweet and sour pork, honey chicken, Mongolian lamb/beef, and fried ice cream.

I have two main theories on why this is. One is that Chinese restaurant food is totally made up, and has no similarity with food anywhere in the world. In which case, I'd better give up trying to find the right location. This seems to be the case with fried ice cream, which Wikipedia suggests is an artificial modern invention.

The other is that maybe I visited the wrong location. China is a big place, after all. Perhaps visiting another area, such as Inner Mongolia, would find me what I'm looking for, with dishes either the same as, or the un-modified version of what is served in western countries' Chinese restaurants.

The Wikipedia article on Chinese restaurant mentions:


  There has additionally been a consequential component of Chinese
  emigration of illegal origin, most notably Fuzhou people from Fujian
  Province and Wenzhounese from Zhejiang Province in Mainland China,
  specifically destined to work in Chinese restaurants in New York City,
  beginning in the 1980s.


As Fujian province is right next to Taiwan, and has had a major influence on it, it seems plausible that if some of the "Chinese restaurant" dishes were coming from Fujian, I would have seen them in Taiwan, but I didn't.

Where, if anywhere, can I find the dishes that feature in western countries' Chinese restaurants? Also, how should I identify the right kind of eatery in those countries?
