\left(
\frac{\partial}{\partial x}
-
\frac{\partial}{\partial y}
\right)
\cdot
\left(
\frac{\partial}{\partial x}
+
\frac{\partial}{\partial y}
\right)
=
\frac{\partial^2}{\partial x^2}
-
\frac{\partial^2}{\partial y^2}
