code: def is_atom(e):
      if isinstance(e, NegatedExpression):
          e = e.term
  
      if isinstance(e, ApplicationExpression):
          for arg in e.args:
              if not TableauProver.is_atom(arg):
                  return False
          return True
      elif isinstance(e, AbstractVariableExpression) or isinstance(
          e, LambdaExpression
      ):
          return True
      else:
          return False
  
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 24 for calling the TableauProver obj's underlying member method code with example usage: obj.is_atom(e) and returns: True, True, False, False