code: def get_logexp(a=1, b=0, a2=None, b2=None, backend=None):
      
      if a2 is None:
          a2 = a
      if b2 is None:
          b2 = b
      if backend is None:
          import sympy as backend
      return (lambda x: backend.log(a*x + b),
              lambda x: (backend.exp(x) - b2)/a2)
docs: Utility function for use with :func:symmetricsys.
  
      Creates a pair of callbacks for logarithmic transformation
      (including scaling and shifting): ``u = ln(a*x + b)``.
  
      Parameters
      ----------
      a : number
          Scaling (forward).
      b : number
          Shift (forward).
      a2 : number
          Scaling (backward).
      b2 : number
          Shift (backward).
  
      Returns
      -------
      Pair of callbacks.