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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.

enter image description here

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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
up vote 5 down vote accepted

Something like this?






b \xrightarrow{b} \surd &
   \quad v:=b) \\

a+b \xrightarrow{b} \surd &
   \quad v:=b, x:=a, y:=b) \\

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

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

enter image description here

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WOW, you are a tex machine :) – tdi Sep 3 '13 at 17:19
@tdi The Joy of TeX! – egreg Sep 3 '13 at 17:25

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