docstr: Pop the first expression that appears in the agenda
code: def pop_first(self):
      """Pop the first expression that appears in the agenda"""
      for i, s in enumerate(self.sets):
          if s:
              if i in [Categories.N_EQ, Categories.ALL]:
                  for ex in s:
                      try:
                          if not ex[0]._exhausted:
                              s.remove(ex)
                              return (ex, i)
                      except AttributeError:
                          s.remove(ex)
                          return (ex, i)
              else:
                  return (s.pop(), i)
      return ((None, None), None)
  
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 7 for calling the Agenda obj's underlying member method code with example usage: obj.pop_first() and returns: None,  None,  None, s,  i, ex,  i, ex,  i