class_docstr: Barrier optimization problem:
      minimize fun(x) - barrier_parameter*sum(log(s))
      subject to: constr_eq(x)     = 0
                constr_ineq(x) + s = 0
code: def get_slack(self, z):
      return z[self.n_vars : self.n_vars + self.n_ineq]
  
imports: import scipy.sparse
  import numpy
  from .equality_constrained_sqp import equality_constrained_sqp
  from scipy.sparse.linalg import LinearOperator
label_desc: Use image node_id 3 for calling the BarrierSubproblem obj's underlying member method code with example usage: obj.get_slack(z) and returns: z