5
\[ \frac{A \rightarrow f[\overrightarrow{B}]:= \Psi \\ \Phi \in
\langle\Psi\rangle} {A \rightarrow f[\bullet\overrightarrow{B}];\Phi} \]

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

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

\Phi \in \langle\Psi\rangle
1
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    – Werner
    Aug 23 '12 at 3:54
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.

1
2

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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