\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ \frac{a}{b} \genfrac{(}{)}{}{}{a}{b} \]

\[ \genfrac{(}{)}{0pt}{}{a}{b}
   \genfrac{(}{)}{1pt}{}{a}{b}
   \genfrac{(}{)}{2pt}{}{a}{b} \]

\newdimen\mythick\mythick=1pt\relax
\[ \genfrac{(}{)}{\mythick}{}{a}{b}
   \genfrac{(}{)}{2\mythick}{}{a}{b}
   \genfrac{(}{)}{3\mythick}{}{a}{b} \]


\[ \genfrac{(}{)}{}{}{a}{b} 
   \genfrac{(}{)}{}{0}{a}{b}
   \genfrac{(}{)}{}{1}{a}{b} 
   \genfrac{(}{)}{}{2}{a}{b} 
   \genfrac{(}{)}{}{3}{a}{b}  \]

\newcount\mystyle\mystyle=0\relax
\[ \genfrac{(}{)}{}{\mystyle}{a}{b}
  \advance\mystyle1  \genfrac{(}{)}{}{\mystyle}{a}{b}
  \advance\mystyle1  \genfrac{(}{)}{}{\mystyle}{a}{b}
  \advance\mystyle1  \genfrac{(}{)}{}{\mystyle}{a}{b}
 \]


\mythick=0pt\relax
\mystyle=3\relax
\[ \genfrac{(}{)}{\mythick}{\mystyle}{a}{b}
  \advance\mythick1pt\relax\advance\mystyle-1\relax\genfrac{(}{)}{\mythick}{\mystyle}{a}{b}
  \advance\mythick1pt\relax\advance\mystyle-1\relax\genfrac{(}{)}{\mythick}{\mystyle}{a}{b}
  \advance\mythick1pt\relax\advance\mystyle-1\relax\genfrac{(}{)}{\mythick}{\mystyle}{a}{b}
 \]

\end{document}