[Orgmode] Big curly bracket

[andrea]

I have to write things like
f(x) = | 0 if x == 1
       | 1 otherwise
       
with the big curly bracket.
It's a common thing and I found in latex is like here
http://www.mathhelpforum.com/math-help/latex-help/59690-left-curly-brace.html
Using \left{ and \begin{gathered}
For some reasons anyway it doens't work.

This 
--8<---------------cut here---------------start------------->8---
    $\phi_{sum}(g1))(\bot) = (\phi_{sum}(g2))(\bot) =$ \newline
    \begin{gathered}
    1 if x < 0 \\
    \bot otherwise \\
    \end{gathered}
--8<---------------cut here---------------end--------------->8---

doesn't to what I'd like it to do.
Any idea?