code: def wave_saver(u, x, y, t):
      """save the wave state for each timestep."""
      global u_hist
      global t_hist
      t_hist.append(t)
      u_hist.append(1.0 * u)
  
imports: import argparse
  import time
  from numpy import sqrt
  import ipyparallel
label_desc: Use image node_id 3 for calling a global function with example usage: wave_saver(u, x, y, t) without return types