  Let $u$ and $v$ be two plurisubharmonic functions in the domain of definition
of the Monge-Amp\`ere operator on a domain $\Omega\subset {\bf C}^n$. We prove
that if $u=v$ on a plurifinely open set $U\subset \Omega$ that is Borel
measurable, then $(dd^cu)^n|_U=(dd^cv)^n|_U$. This result was proved by Bedford
and Taylor in the case where $u$ and $v$ are locally bounded, and by El Kadiri
and Wiegerinck when $u$ and $v$ are finite, and by Hai and Hiep when $U$ is of
the form $U=\bigcup_{j=1}^m\{\varphi_j>\psi_j\}$, where $\varphi_j$, $\psi_j$,
$j=1,...,m$, are plurisubharmonic functions on $\Omega$.
