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\[ \frac{A \rightarrow f[\overrightarrow{B}]:= \Psi \\ \Phi \in
\langle\Psi\rangle} {A \rightarrow f[\bullet\overrightarrow{B}];\Phi} \]

I need to get the two antecedents in two seperate lines.

A \rightarrow f[\overrightarrow{B}]:= \Psi

\Phi \in \langle\Psi\rangle
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migrated from stackoverflow.com Aug 23 '12 at 3:50

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Welcome to TeX.sx! Your question was migrated here from Stack Overflow. Please register on this site, too, and make sure that both accounts are associated with each other (by using the same OpenID), otherwise you won't be able to comment on or accept answers or edit your question. –  Werner Aug 23 '12 at 3:54

2 Answers 2

Although I am unfamiliar with the context, you can use an array to brake the "antecedents" into separate lines:

enter image description here

\documentclass{article}
\begin{document}
\[
  \frac{\begin{array}{@{}c@{}}
    A \rightarrow f[\overrightarrow{B}]:= \Psi \\
      \Phi \in \langle\Psi\rangle
  \end{array}}{
    A \rightarrow f[\bullet\overrightarrow{B}];\Phi}
\]
\end{document}

This will place the object centred vertically around the fraction line. If that is not of concern, you can do everything inside an array environment:

\[
  \begin{array}{c}
    A \rightarrow f[\overrightarrow{B}]:= \Psi \\
    \Phi \in \langle\Psi\rangle \\
    \hline
    A \rightarrow f[\bullet\overrightarrow{B}];\Phi
  \end{array}
\]

There are other packages that also provide inference rule functionality, like semantic or proofs.

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I think you meant the proofs package –  Geoff Reedy Aug 23 '12 at 13:10
    
@GeoffReedy: Thanks. –  Werner Aug 23 '12 at 14:30

I have found the mathpartir package the easiest to use for typesetting inference rules in this style. The default style is side-by-side antecedents but you can put them on separate lines by putting two line breaks between the antecedents. In this case

\[
\inferrule{A \rightarrow f[\overrightarrow{B}]:= \Psi \\\\
           \Phi \in \langle\Psi\rangle}
          {A \rightarrow f[\bullet\overrightarrow{B}];\Phi}
\]

which will look like

typeset example

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