  ```
import argparse
import sys
import numpy as np
import matplotlib.pyplot as plt
import scipy.sparse as sp
from mumps import Mumps
from fealpy import LagrangeFiniteElementSpace, UniformTimeLine, TriangleMesh

# Define the Poisson PDE
def poisson_pde(u, v, x, y):
    return u.grad(x, y) * v

# Define the Navier-Stokes PDE
def navier_stokes_pde(u, v, p, x, y):
    return u.grad(x, y) * v + u * v.grad(x, y) - p.grad(x, y)

# Define the command-line argument parser
parser = argparse.ArgumentParser()
parser.add_argument('--motion_degree', type=int, default=1)
parser.add_argument('--pressure_degree', type=int, default=1)
parser.add_argument('--time_divisions', type=int, default=10)
parser.add_argument('--end_time', type=float, default=1.0)
parser.add_argument('--output_dir', type=str, default='output')
parser.add_argument('--steps', type=int, default=100)
parser.add_argument('--nonlinear_method', type=str, default='newton')

# Parse the arguments
args = parser.parse_args()

# Define the motion and pressure finite element spaces
motion_space = LagrangeFiniteElementSpace(TriangleMesh, args.motion_degree)
pressure_space = LagrangeFiniteElementSpace(TriangleMesh, args.pressure_degree)

# Define the time line
time_line = UniformTimeLine(args.time_divisions, args.end_time)

# Define the bilinear form and mixed bilinear form
bilinear_form = BilinearForm(motion_space, pressure_space)
mixed_bilinear_form = MixedBilinearForm(motion_space, pressure_space)

# Add domain integrators to the bilinear form
bilinear_form.add_domain_integrator(DomainIntegrator(poisson_pde))
mixed_bilinear_form.add_domain_integrator(DomainIntegrator(navier_stokes_pde))

# Assemble the forms and get their matrices
A = bilinear_form.assemble()
M = mixed_bilinear_form.assemble()

# Calculate the mass matrix of the motion space
mass_matrix = motion_space.get_mass_matrix()

# Initialize the error matrix
error_matrix = np.zeros((motion_space.get_number_of_dofs(), pressure_space.get_number_of_dofs()))

# Loop over time steps
for i in range(args.steps):
    # Advance to the next time level
    time_line.advance()

    # Add a scalar convection integrator to the bilinear form
    bilinear_form.add_domain_integrator(ScalarConvectionIntegrator(time_line.get_current_time()))

    # Assemble the bilinear form and get its matrix
    A_new = bilinear_form.assemble()

    # Calculate the divergence matrix and a new matrix M
    divergence_matrix = motion_space.get_divergence_matrix()
    M_new = M + divergence_matrix

    # Calculate the source vector and set up boundary conditions
    source_vector = np.zeros(motion_space.get_number_of_dofs())
    boundary_conditions = [DirichletBC(motion_space, 0, 'on_boundary')]

    # Solve the system of equations
    solution = M_new.solve(source_vector, boundary_conditions)

    # Update the motion and pressure functions
    u_new = solution[0]
    p_new = solution[1]

    # Calculate the L2 error and maximum error
    error = np.linalg.norm(u_new - u_old)
    max_error = np.max(np.abs(u_new - u_old))

    # Print the sum of absolute values of the motion function
    print(np.sum(np.abs(u_new)))

    # Update the old motion and pressure functions
    u_old = u_new
    p_old = p_new

# Plot the motion function
plt.plot(u_new)
plt.xlabel('x')
plt.ylabel('u')
plt.show()
```