code: def fast_relpath_optional(path, start):
    
    if len(start) == 0:
      # Empty prefix.
      return path
  
    # Determine where the matchable prefix ends.
    pref_end = len(start) - 1 if start[-1] == '/' else len(start)
    if pref_end > len(path):
      # The prefix is too long to match.
      return None
    elif path[:pref_end] == start[:pref_end] and (len(path) == pref_end or path[pref_end] == '/'):
      # The prefix matches, and the entries are either identical, or the suffix indicates that
      # the prefix is a directory.
      return path[pref_end+1:]
docs: A prefix-based relpath, with no normalization or support for returning `..`.
  
    Returns None if `start` is not a directory-aware prefix of `path`.