[pstricks] Unbalanced trees with pstree

[Mårten Trolin]

Hello,

I want to make a three-level tree where the nodes on the second level 
have different number of sons. The code I use is as follows

\pstree[]
          { \TR{$M_\rho$}}
           { \pstree{ \TR{$M_{\beta_1}$}}
               {\TR{$S_{\alpha_1}$} \TR{$S_{\alpha_2}$} }
             \pstree{ \TR{$M_{\beta_2}$}}
               {\TR{$S_{\alpha_3}$} \TR{$S_{\alpha_4}$}
                 \TR{$S_{\alpha_5}$} \TR{$S_{\alpha_6}$} }
             \pstree{ \TR{$M_{\beta_3}$}}
               {\TR{$S_{\alpha_7}$} \TR{$S_{\alpha_8}$}
                 \TR{$S_{\alpha_9}$} }
          }

I want the subtrees to be evenly spaced (e.g., I want the edge between 
M_\rho and M_{\beta_2} to be vertical). I have experimented with
treenodesize, thistreenodesize and treesep without any luck. Any ideas 
are very welcome!

Mårten