I am new to fenics am trying to solve 1D heat equation. i used theta method for time discretization and i am encountering error solving the variational problem. i would be grateful if i could get some help.

here

```
from fenics import *
import numpy as np
import matplotlib.pyplot as plt
from dolfin import *
from ufl import *
from fenics import *
import math
# Create mesh and define function space
nx = ny = 32
mesh = UnitSquareMesh(nx, ny)
V = FunctionSpace(mesh, 'CG', 1)
theta = Constant(0.5)
dt = Constant(0.1)
num_steps = 10
# Define boundary condition
u_D = Expression('sin(2*pi*x[0])', degree = 2)
def GammaD(x, on_boundary):
return near(x[1], 0)
g = Constant('0')
bc = DirichletBC(V, g, GammaD)
#
#u_n = project(u_D, V)
# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
u_n = Function(V)
f = Constant(0)
F = (u*v -theta*dt*inner(grad(u),grad(v)))*dx*u_n - (u*v-(1-theta)*dt*inner(grad(u),grad(v)))*u*dx
a, L = lhs(F), rhs(F)
#Define initial value
u_n = interpolate(u_D, V)
# Time-stepping
u = Function(V)
t = 0
for n in range(num_steps):
# Update current time
t += dt
u_D.t = t
# Compute solution
solve(a == L, u, bc)
# Plot solution
plot(u)
# Compute error at vertices
u_e = interpolate(u_D, V)
error = np.abs(u_e.vector().array() - u.vector().array()).max()
print('t = %.2f: error = %.3g' % (t, error))
# Update previous solution
u_n.assign(u)
# Hold plot
interactive()
```

is the code

And error

```
NotImplementedError Traceback (most recent call last)
<ipython-input-13-f382af29c6b6> in <module>
52
53 # Compute solution
---> 54 solve(a == L, u, bc)
55
56 # Plot solution
```