Notation for arguments by means of ifthen package

Question

I have this idea to create a piece of code that would allow me for a handy introduction of arguments in my notes for students, by which I mean this kind of fractional notation:

I need a command that would adjust the horizontal line by measuring premises and conclusion, and would add 0.5em (or whatever) on each side beyond the sentences. Using ifthen package I came up with this very imperfect (to put it mildly) three-argument \inference command:

\newlength{\premiseone}
\newlength{\premisetwo}
\newlength{\conclusion}
\newcommand{\argument}[3]{%
\settowidth{\premiseone}{#1}
\settowidth{\premisetwo}{#2}
\settowidth{\conclusion}{#3}
\addtolength{\premiseone}{1em}
\addtolength{\premisetwo}{1em}
\addtolength{\conclusion}{1em}
\ifthenelse{\lengthtest{\premiseone > \premisetwo}}%
{\ifthenelse{\lengthtest{\premiseone > \conclusion}}%
{\dfrac{\parbox{\premiseone}{\center #1\\#2}}{\mbox{#3}}}%
{\dfrac{\parbox{\conclusion}{\center #1\\#2}}{\mbox{#3}}}%
}

It is specifically designed for arguments with exactly two premises and has rather unwelcome features because of it. For example, if I write, e.g., \argument{the only premise}{}{the consequence}, LaTeX complains that there is no line to end, and If I add some empty box I have this unwanted additional space, either above the argument or inside it:

Here are my questions to you:

1. How could I get round the problem of additional spaces? That is, how could I skip some arguments by writing \argument{the only premise}{}{the consequence} without obtaining this additional vertical space (either above or inside)?
2. Is there any relatively easy way to build a command which would not be designed for a specific number of premises, but would work in a similar way as mine yet for any finite number of premises? Of course, I could always build \argumentone, \arguemtttwo, \argumentthree, but this is going round the houses.

Answer 1

I would go with a simple tabular:

\documentclass{article}

\newcommand*{\inference}[3][t]{%
   \begingroup
   \def\and{\\}%
   \begin{tabular}[#1]{@{\enspace}c@{\enspace}}
   #2 \\
   \hline
   #3
   \end{tabular}%
   \endgroup
}

\begin{document}

First example:
\inference{John is taller than Jane}{John is taller than Judy}

\bigskip

Second example:
\inference{John is taller than Jane \and Jane is taller than Judy}{John is taller than Judy}

\bigskip

Third example (vertically centred):
\inference[c]{John is taller than Jane \and Jane is taller than Judy \and Whatever else}{John is taller than Judy}

\end{document}

I added an optional argument for vertical placing. Of course, further additional space above/below the tabular can be added (that will depend on how this thing is used in a real document).