code: def generate(grammar, start=None, depth=None, n=None):
      """
      Generates an iterator of all sentences from a CFG.
  
      :param grammar: The Grammar used to generate sentences.
      :param start: The Nonterminal from which to start generate sentences.
      :param depth: The maximal depth of the generated tree.
      :param n: The maximum number of sentences to return.
      :return: An iterator of lists of terminal tokens.
      """
      if not start:
          start = grammar.start()
      if depth is None:
          depth = sys.maxsize
  
      iter = _generate_all(grammar, [start], depth)
  
      if n:
          iter = itertools.islice(iter, n)
  
      return iter
  
imports: import itertools
  import sys
  from nltk.grammar import Nonterminal
label_desc: Use image node_id 1 for calling a global function with example usage: generate(grammar, start, depth, n) and returns: iter