input: Write a function that accepts a string of space-separated lowercase letters and returns a dictionary with the most frequently occurring letters and their counts. If multiple letters have the highest frequency, include them all.
  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):
      letters = s.split()
      if not letters:
          return {}
      freq = {}
      for letter in letters:
          if letter in freq:
              freq[letter] += 1
          else:
              freq[letter] = 1
      max_freq = max(freq.values())
      return {letter: count for letter, count in freq.items() if count == max_freq}