##
12.5.1 MultiFile Objects

A `MultiFile` instance has the following methods:

`readline`(str)
  Read a line. If the line is data (not a section-divider or end-marker or real EOF) return it. If the line matches the most-recently-stacked boundary, return `''` and set `self.last` to 1 or 0 according as the match is or is not an end-marker. If the line matches any other stacked boundary, raise an error. On encountering end-of-file on the underlying stream object, the method raises `Error` unless all boundaries have been popped.

`readlines`(str)
  Return all lines remaining in this part as a list of strings.

`read`()
  Read all lines, up to the next section. Return them as a single (multiline) string. Note that this doesn't take a size argument!

`seek`(pos[, whence])
  Seek. Seek indices are relative to the start of the current section. The pos and whence arguments are interpreted as for a file seek.

`tell`()
  Return the file position relative to the start of the current section.

`next`()
  Skip lines to the next section (that is, read lines until a section-divider or end-marker has been consumed). Return true if there is such a section, false if an end-marker is seen. Re-enable the most-recently-pushed boundary.

`is_data`(str)
  Return true if str is data and false if it might be a section boundary. As written, it tests for a prefix other than `'-``-'` at start of line (which all MIME boundaries have) but it is declared so it can be overridden in derived classes.

  Note that this test is used intended as a fast guard for the real boundary tests; if it always returns false it will merely slow processing, not cause it to fail.

`push`(str)
  Push a boundary string. When an appropriately decorated version of this boundary is found as an input line, it will be interpreted as a section-divider or end-marker. All subsequent reads will return the empty string to indicate end-of-file, until a call to `pop()` removes the boundary a or `next()` call reenables it.

  It is possible to push more than one boundary. Encountering the most-recently-pushed boundary will return EOF; encountering any other boundary will raise an error.

`pop`()
  Pop a section boundary. This boundary will no longer be interpreted as EOF.

`section_divider`(str)
  Turn a boundary into a section-divider line. By default, this method prepends `'-``-'` (which MIME section boundaries have) but it is declared so it can be overridden in derived classes. This method need not append LF or CR-LF, as comparison with the result ignores trailing whitespace.

`end_marker`(str)
  Turn a boundary string into an end-marker line. By default, this method prepends `'-``-'` and appends `'-``-'` (like a MIME-multipart end-of-message marker) but it is declared so it can be be overridden in derived classes. This method need not append LF or CR-LF, as comparison with the result ignores trailing whitespace.

Finally, `MultiFile` instances have two public instance variables:

`level`
  Nesting depth of the current part.

`last`
  True if the last end-of-file was for an end-of-message marker.