class_docstr: asymmetric mixed model of Tawn 1988
  
  special case:  k=0, theta in [0,1] : symmetric mixed model of
      Tiago de Oliveira 1980
  
  restrictions:
   - theta > 0
   - theta + 3*k > 0
   - theta + k <= 1
   - theta + 2*k <= 1
code: def evaluate(self, t, theta, k):
      transf = 1 - (theta + k) * t + theta * t * t + k * t**3
      return transf
  
imports: import numpy
  from scipy import stats
  from statsmodels.tools.numdiff import _approx_fprime_cs_scalar, approx_hess
label_desc: Use image node_id 2 for calling the AsymMixed obj's underlying member method code with example usage: obj.evaluate(t, theta, k) and returns: transf