code: def split_header(line):
          
          match = re.match(r'^(CF\.\{.*?}): (.*)$', line)
          if match:
              return (match.group(1), match.group(2))
          return line.split(': ', 1)
docs: Split a header line into field name and field value.
  
          Note that custom fields may contain colons inside the curly braces,
          so we need a special test for them.
  
          :param line: A message line to be split.
  
          :returns: (Field name, field value) tuple.