\begin{center}
\begin{forest}
for tree={circle, draw,
            minimum size=2em,
            inner sep=0pt,
            s sep=2mm,
            l sep=1mm}
	[$\imp$
		[$\lor$
			[$a$]
			[$b$]
		]
		[$\neg$
			[$x$
			  [$\neg$]
      ]
		]
  ]
\end{forest}
\end{center}

One leaf is labeled with a logical operator, which is not allowed.
Thus this is not a well-formed formula.
