input: Write a function that takes a string of space-separated lowercase letters and returns a dictionary with the most frequently occurring letters and their counts. If multiple letters occur equally often, 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
      if not s:
          return {}
      counter = Counter(s.replace(' ', ''))
      max_count = max(counter.values())
      return {k: v for k, v in counter.items() if v == max_count}