def METHOD_NAME(self, key, value, prefix=0):
    """Add a header line to the MIME message.
    The key is the name of the header, where the value obviously provides
    the value of the header. The optional argument prefix determines
    where the header is inserted; 0 means append at the end, 1 means
    insert at the start. The default is to append.
    """
    lines = value.split("\n")
    while lines and not lines[-1]: del lines[-1]
    while lines and not lines[0]: del lines[0]
    for i in range(1, len(lines)):
        lines[i] = "    " + lines[i].strip()
    value = "\n".join(lines) + "\n"
    line = key + ": " + value
    if prefix:
        self._headers.insert(0, line)
    else:
        self._headers.append(line)