Let $X$ be a smooth manifold, let $m \in \mathbb{R}$, and let $P \in \Psi^m_{\mathrm{prop}}(X)$ be a properly supported classical pseudodifferential operator of order $m$. Let
be the real homogeneous principal symbol of $P$, homogeneous of degree $m$ in the cotangent variable. Let $H_p$ denote the Hamilton vector field of $p$ on $T^*X \setminus 0$ with respect to the canonical symplectic form, and let $\Phi_t$ denote its local Hamilton flow wherever defined. Let $\Omega \subset T^*X \setminus 0$ be an open conic set, and define
be a compact bicharacteristic segment of $H_p$, meaning that $s_0 < s_1$, $\gamma$ is a $C^1$ map, and $\gamma'(s)=H_p(\gamma(s))$ for every $s \in (s_0,s_1)$. Suppose that
and this set is invariant under the bicharacteristic flow in the following local sense: if $J \subset \mathbb{R}$ is an interval, $\eta:J\to\Sigma_\Omega$ is a bicharacteristic of $H_p$ with $\eta(J)\subset\Omega$, and $K\subset J$ is a compact interval such that $\eta(K)\cap\operatorname{WF}(Pu)=\varnothing$, then membership of $\eta(s)$ in $\operatorname{WF}(u)$ is constant for $s\in K$.