input: Write a python function that receives a string containing space-separated lowercase letters and returns a dictionary that lists the most frequently appearing letters and their counts. If there is a tie for the most frequent letters, include all of them.
  assert histogram('a b b a') == {'a':2,'b': 2}
  assert histogram('a b c a b') == {'a': 2, 'b': 2}
  assert histogram('a b c d g') == {'a': 1, 'b': 1, 'c': 1, 'd': 1, 'g': 1}
  assert histogram('r t g') == {'r': 1,'t': 1,'g': 1}
  assert histogram('b b b b a') == {'b': 4}
  assert histogram('r t g') == {'r': 1,'t': 1,'g': 1}
  assert histogram('') == {}
  assert histogram('a') == {'a': 1}
output: def histogram(s):
      from collections import Counter
      counts = Counter(s.split())
      max_count = max(counts.values(), default=0)
      return {k: v for k, v in counts.items() if v == max_count}