Hello,

I am trying to implement a flux BC on the Poisson’s equation for the top boundary and then measure the flux at the bottom boundary, I received a warning says that

Notation dx[meshfunction] is deprecated. Please use dx(subdomain_data=meshfunction) instead.

I didn’t use `dx[meshfunction]`

in my code but I still received this warning, so I am concerned if my implementation was incorrect; also, the out flux on the bottom edge I computed is the same as the applied flux on the top boundary, this seems odd. following is my code:

```
from dolfin import *
mesh = UnitSquareMesh(32, 32)
V = FunctionSpace(mesh, "Lagrange", 2)
def bot(x):
return x[1] < DOLFIN_EPS
u0 = Constant(20.0)
bc_bot = DirichletBC(V, u0, bot)
boundary_parts = MeshFunction("size_t", mesh, mesh.topology().dim()-1, 0)
boundary_parts.set_all(0) #marks whole cube as domain 0
class Top(SubDomain):
def inside(self, x, on_boundary):
return on_boundary and x[1] > 1.0 - DOLFIN_EPS
top = Top()
top.mark(boundary_parts, 1) #marks the top of the cube as 1
class Bot(SubDomain):
def inside(self, x, on_boundary):
return on_boundary and x[1] < DOLFIN_EPS
bottom = Bot()
bottom.mark(boundary_parts, 2) #marks the bottom of the cube as 2
ds=Measure('ds')[boundary_parts]
u = TrialFunction(V)
v = TestFunction(V)
f = Constant(0.0)
g = Constant(-100.0) # in flux, negative
alpha = 10. #thermal diffusivity
a = Constant(alpha)*inner(grad(u), grad(v))*dx
L = f*v*dx - g*v*ds(1)
u = Function(V)
solve(a == L, u, bc_bot)
outflux = assemble(Constant(alpha)*grad(u)[1]*ds(2))
print('outflux = ', outflux)
```

I appreciate any advice on this! Thank you!