class_docstr: t-EV model of Demarta and McNeil 2005
  
  restrictions:
   - rho in (-1,1)
   - x > 0
code: def _check_args(self, rho, df):
      x = df  # alias, Genest and Segers use chi, copual package uses df
      cond1 = x > 0
      cond2 = (rho > 0) and (rho < 1)
      return cond1 and cond2
  
imports: import numpy
  from scipy import stats
  from statsmodels.tools.numdiff import _approx_fprime_cs_scalar, approx_hess
label_desc: Use image node_id 1 for calling the TEV obj's underlying member method code with example usage: obj._check_args(rho, df) and returns: unknown