text: import numpy as np
  import lmfit
  
  class SinglePoleStep:
      def __init__(self, amp, tau):
          self.amp_s = amp
          self.tau_s = tau
          self.amp_k = self.amp_s/(self.amp_s-1.)
          self.tau_k = (1.-self.amp_s)*self.tau_s
  
          self.step_func = lambda t: 1.-self.amp_s*np.exp(-t/self.tau_s)
  
          self.kernel_step_func = lambda t: 1.+self.amp_k*np.exp(-t/self.tau_k)
          self.step_model = lmfit.Model(self.step_func)
          self.kernel_step_model = lmfit.Model(self.kernel_step_func)