  We provide a direct combinatorial proof of a Feynman graph identity which
implies a wide generalization of a formality theorem by Kontsevich. For a
Feynman graph $\Gamma$, we associate to each vertex a position $x_v \in \mathbb
R$ and to each edge $e$ the combination $s_e = a_e^{-\frac 12} \left( x^+_e -
x^-_e \right)$, where $x^\pm_e$ are the positions of the two end vertices of
$e$ and $a_e$ a Schwinger parameter. The $``$topological propagator$"$ $P_e =
e^{-s_e^2}$d$s_e$ includes a part proportional to d$x_v$ and a part
proportional to d$a_e$. Integrating the product of all $P_e$ over positions
produces a differential form $\alpha_\Gamma$ in the variables $a_e$. We prove
that $\alpha_\Gamma \wedge \alpha_\Gamma=0$.
