# Create multiline fraction

How to create an equation like in the image below. I tried to use \cfraq and \hline but with no success. I am asking for the layout only, not the formulae.

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What should be the rule governing the separator lines in the left column? –  egreg Sep 3 '13 at 12:57
@egreg I am afraid I do no understand the question, could you paraphrase ? They should be as long as the equation below them. –  tdi Sep 3 '13 at 13:01
That was the question. ;-) –  egreg Sep 3 '13 at 13:01
@egreg I can easily recreate this with align I just do not know how to add these lines. –  tdi Sep 3 '13 at 13:03
Why don't you post the code that you have to this point, for us to use a starting point? –  Steven B. Segletes Sep 3 '13 at 13:52

Something like this?

\documentclass{article}
\usepackage{amsmath,array,booktabs,collcell}

\newenvironment{productions}
{\begin{equation*}
\begin{array}{
>{\collectcell\firstcolumn}c<{\endcollectcell}
>{\displaystyle}l
}
}
{\end{array}
\global\firstcelltrue
\end{equation*}}

\newif\iffirstcell
\firstcelltrue

\newcommand{\firstcolumn}[1]{%
\begin{tabular}[b]{c}
\iffirstcell
\global\firstcellfalse
\else
\fi
$\displaystyle#1$
\end{tabular}
}

\begin{document}

\begin{productions}
b \xrightarrow{b} \surd &
(\frac{}{v\xrightarrow{v}\surd},

a+b \xrightarrow{b} \surd &
(\frac{y\xrightarrow{v}\surd}{x+y\xrightarrow{v}\surd},

(a+b)\cdot c \xrightarrow{b} c &
(\frac{x\rightarrow{v}\surd}{x\cdot y\xrightarrow{v}y},

((a+b)\cdot c)\cdot d \xrightarrow{b} c\cdot d &
(\frac{x\rightarrow{v}x'}{x\cdot y\xrightarrow{v}x'\cdot y},