begin{forest}
[$root : s_0$
[$a: s_*$
[$d: s_*$]
[$d: s_*$
[$c: s_*$]
]
]
[$a: s_a$
[$b: s_b$
[$c: s_c$]
]
[$b: s_*$
[$d: s_*$]
[$c: s_*$
[$d: s_*$
[$d: s_*$]
]
]
]
]
]
end{forest}