pde(u) [+-] op1(u)[*/]exp1 [+-] op2(u)[*/]exp2...=exp3
It defines a partial differential equation with non constant
coefficients where op is one of the operator:
id()
dx()
dy()
laplace()
dxx()
dxy()
dyx()
dyy()
[*/] means either a * or a / and similarly for ±.
Note that the expressions are necessarily AFTER the operator while in
practice they are between the 2 differentiations for laplace...dyy .
Thus laplace(u)*(x+y) means

.Similarly dxy(u)*f means
