code: def event_fn(self, t, state):
      # positive if ball in mid-air, negative if ball within ground.
      pos, _, log_radius = state
      return pos - torch.exp(log_radius)
  
file_path: torchdiffeq/examples/bouncing_ball.py
imports: import argparse
  import matplotlib.pyplot
  import torch
  import torch.nn
  from torchdiffeq import odeint, odeint_adjoint
  from torchdiffeq import odeint_event
label_desc: Use image node_id 3 for calling the BouncingBallExample obj's underlying member method code with example usage: obj.event_fn(t, state) and returns: unknown