code: def project(vx, vy):
      """Project the velocity field to be approximately mass-conserving,
      using a few iterations of Gauss-Seidel."""
      p = np.zeros(vx.shape)
      h = 1.0 / vx.shape[0]
      div = (
          -0.5
          * h
          * (
              np.roll(vx, -1, axis=0)
              - np.roll(vx, 1, axis=0)
              + np.roll(vy, -1, axis=1)
              - np.roll(vy, 1, axis=1)
          )
      )
  
      for k in range(10):
          p = (
              div
              + np.roll(p, 1, axis=0)
              + np.roll(p, -1, axis=0)
              + np.roll(p, 1, axis=1)
              + np.roll(p, -1, axis=1)
          ) / 4.0
  
      vx -= 0.5 * (np.roll(p, -1, axis=0) - np.roll(p, 1, axis=0)) / h
      vy -= 0.5 * (np.roll(p, -1, axis=1) - np.roll(p, 1, axis=1)) / h
      return vx, vy
  
file_path: autograd/examples/fluidsim/fluidsim.py
imports: from __future__ import absolute_import
  from __future__ import print_function
  from builtins import range
  import autograd.numpy
  from autograd import value_and_grad
  from scipy.optimize import minimize
  from scipy.misc import imread
  import matplotlib
  import matplotlib.pyplot
  import os
label_desc: Use image node_id 1 for calling a global function with example usage: project(vx, vy) and returns: vx,  vy