code: def dfunc(pars, x, data=None):
      a, b = pars["a"], pars["b"]
      v = np.exp(-b * x)
      return np.array([v, -a * x * v, np.ones(len(x))])
  
imports: import matplotlib.pyplot
  import numpy
  from lmfit import Minimizer, Parameters
label_desc: Use image node_id 2 for calling a global function with example usage: dfunc(pars, x, data) and returns: np