  Heisenberg's position-measurement--momentum-disturbance relation is derivable
from the uncertainty relation $\sigma(q)\sigma(p) \geq \hbar/2$ only for the
case when the particle is initially in a momentum eigenstate. Here I derive a
new measurement--disturbance relation which applies when the particle is
prepared in a twin-slit superposition and the measurement can determine at
which slit the particle is present. The relation is $d \times \Delta p \geq
2\hbar/\pi$, where $d$ is the slit separation and $\Delta p=D_{M}(P_{f},P_{i})$
is the Monge distance between the initial $P_{i}(p)$ and final $P_{f}(p)$
momentum distributions.
