proof in Lyx

[Nicolas SABOURET]

How can I make inferences like :
   B C
   --- (y)
 A  D
------ (x)
  E

There exists a "proof.sty" package with a "\infer{}{}" command, but if
A,B,C,D,E are big math formulaes, I can't use "\infer{}{}" in math mode
: it doesn't hold on a single lyx line (screen isn't wide enough :)). Is
there a solution ?
Can I temporarily redefine \frac by \infer ? How can I do this ?
Nicolas.