code: def to_mixed_case(s):
      
      out = ''
      last_c = ''
      for c in s:
          if c == '_':
              pass
          elif last_c in ('', '_'):
              out += c.upper()
          else:
              out += c.lower()
          last_c = c
      return out
docs: convert upper snake case string to mixed case, e.g. MIXED_CASE becomes
      MixedCase