def count(s):
    """Count three things in a sentence and return a dict.

    Keys:
      "length"  — total characters in s (including spaces)
      "words"   — number of whitespace-separated words
      "a_count" — count of lowercase 'a's after lowercasing s

    Helpers:
      len(s)             — char count
      len(s.split())     — word count
      s.lower().count("a") — number of 'a's, case-insensitive

    count("Hello andromeda") -> {"length": 15, "words": 2, "a_count": 2}
    """
    pass