docstr: ...Test HawkesKernelExp string representation
          
code: def test_HawkesKernelExp_str(self):
      """...Test HawkesKernelExp string representation"""
      intensity = 3
      decay = 2
      hawkes_kernel_exp = HawkesKernelExp(intensity, decay)
      self.assertEqual(str(hawkes_kernel_exp), "3 * 2 * exp(- 2 * t)")
  
      intensity = 3
      decay = 0
      hawkes_kernel_exp = HawkesKernelExp(intensity, decay)
      self.assertEqual(str(hawkes_kernel_exp), "3")
  
      intensity = 0
      decay = 2
      hawkes_kernel_exp = HawkesKernelExp(intensity, decay)
      self.assertEqual(str(hawkes_kernel_exp), "0")
  
file_path: tick/tick/hawkes/simulation/tests/hawkes_kernel_exp_test.py
imports: import unittest
  from tick.hawkes import HawkesKernelExp
label_desc: Use image node_id 4 for calling the Test obj's underlying member method code with example usage: obj.test_HawkesKernelExp_str() without return types