Putting a brace across multiple lines with text in Plain TeX

Question

I'm changing a document prepared in Plain TeX and I'm trying to get a brace to cover multiple lines for example:

S is P { cannot both be true (L. of Contradiction)
S is not P { cannot both be false (L. of Excluded Middle)

I know how to do this in LaTeX but I am struggling to find a Plain TeX version. I have this so far which causes excessive whitespace and pushes the RHS almost off the page.

$ \vbox{ {\it S is P}\hfil\break
         {\it S is not P} } \left\{ \vbox{cannot both be true (L. of Contradiction)\hfil \break
                                         cannot both be false (L. of Excluded Middle).} \right. $

Does anyone have any suggestions? Thankyou.

Answer 1

You can use predefined structures:

$$
\matrix{
  \hbox{\it S is P}\hfill\cr
  \hbox{\it S is not P}
}\quad
\cases{
  \hbox{cannot both be true (L. of Contradiction)}\cr
  \hbox{cannot both be false (L. of Excluded Middle)}
}
$$

\bye

Answer 2

Your problem is that you started paragraph mode inside both \vboxes, so they have \hsize width which is not reduced. If you want a text inside a \vbox without starting paragraph mode, put the text into \hbox.

Moreover, you want vertically aligned result: \vcenter in math mode should be used instead \vbox.

$$ 
  \vcenter{ \hbox{\it S is P}
            \hbox{\it S is not P}
  } \quad \left\{
  \vcenter{ \hbox{cannot both be true (L. of Contradiction)}
            \hbox{cannot both be false (L. of Excluded Middle)}
  } \right.
$$

\bye

Answer 3

Like this?

\noindent
$$\left\{\vcenter{\halign{\it #\hfill &\quad #\hfill\cr
S is P & cannot both be true (L. of Contradiction)\cr
S is not P & cannot both be false (L. of Excluded Middle)\cr}}\right.$$
\bye

or perhaps like this?

\noindent
$$\vcenter{\halign{\it #\hfill\cr
S is P\cr
S is not P\cr}}\quad
\left\{\vcenter{\halign{#\hfill\cr
cannot both be true (L. of Contradiction)\cr
cannot both be false (L. of Excluded Middle)\cr}}\right.$$
\bye