Je me suis battu en vain avec pstplus
et finalement la documentation de pstree est largement suffisante.
\psset{treemode=R,nodesep=1mm,levelsep=20mm,treesep=5mm}
%\pstree[treemode=R,levelsep=15ex]{\Tcircle{R} }{
\pstree{\TR{$\Omega$} }{
\pstree{ \Tcircle{$A1$}\taput{$\frac18$} }
{
\pstree{ \Tcircle{$A2$}\taput{$\frac18$} }
{
\Tcircle{$A3$}\taput{$\frac18$}
\Tcircle{$\overline{A3}$}\tbput{$\frac78$}
}
\pstree{ \Tcircle{$\overline{A2}$}\taput{$\frac78$} }
{
\Tcircle{$A3$}\taput{$\frac18$}
\Tcircle{$\overline{A3}$}\tbput{$\frac78$}
}
}
\pstree{ \Tcircle{$\overline{A1}$}\taput{$\frac78$} }
{
\pstree{ \Tcircle{$A2$}\taput{$\frac18$} }
{
\Tcircle{$A3$}\taput{$\frac18$}
\Tcircle{$\overline{A3}$}\tbput{$\frac78$}
}
\pstree{ \Tcircle{$\overline{A2}$}\taput{$\frac78$} }
{
\Tcircle{$A3$}\taput{$\frac18$}
\Tcircle{$\overline{A3}$}\tbput{$\frac78$}
}
}
}