\item \BDDToDNF

\begin{center}
\begin{forest}
for tree={circle, draw, no edge,
          minimum size=2em,
          inner sep=0pt,
          s sep=10mm,
          l sep=4mm}
[$f$, rectangle, draw,  tikz={\draw [fulldot=.5] () to (c.north);}
   [$c$, name=c, tikz={\draw [line] () to (d1.north); \draw [dot=.5] () to (d2.north);}
      [$d$, name=d1, tikz={\draw [line] () to (b.north); \draw [dot=.5] () to (266:2.7);}
         [$b$, name=b, tikz={\draw [line] () to (a.north); \draw [fulldot=.5] () to (266:3.8);}
            [$a$, name=a, tikz={\draw [line] () to (256:5.2); \draw [fulldot=.5] () to (266:5);}]
         ]
      ]
      [$d$, name=d2, tikz={\draw [line] () to (b.east); \draw [dot=.5] () to (a.east);}]
   ]
]
\end{forest}
\end{center}