code: def _attempt_proof_iff(
      self, current, context, agenda, accessible_vars, atoms, debug
  ):
      new_agenda = agenda.clone()
      agenda.put(current.first, context)
      agenda.put(current.second, context)
      new_agenda.put(-current.first, context)
      new_agenda.put(-current.second, context)
      return self._attempt_proof(
          agenda, accessible_vars, atoms, debug + 1
      ) and self._attempt_proof(
          new_agenda, accessible_vars, atoms, debug + 1
      )
  
imports: from nltk.inference.api import BaseProverCommand, Prover
  from nltk.internals import Counter
  from nltk.sem.logic import AbstractVariableExpression, AllExpression, AndExpression, ApplicationExpression, EqualityExpression, ExistsExpression, Expression, FunctionVariableExpression, IffExpression, ImpExpression, LambdaExpression, NegatedExpression, OrExpression, Variable, VariableExpression, unique_variable
label_desc: Use image node_id 19 for calling the TableauProver obj's underlying member method code with example usage: obj._attempt_proof_iff(current, context, agenda, accessible_vars, atoms, debug) and returns: unknown