
 Definition of a foldable part of the document
(may be folded in or out at will by the reader):
up to 3 arguments. 
 
- Argument 1: either the name of another block of the same document, or
 empty.
 
- Argument 2: a text identifying the foldable part. If argument 1 is
 another block, this argument can be empty (in this case the text will
 default to the title of the cited block).
 
- Argument 3: only used when argument 1 is empty, giving the content of
 the foldable part (when it is folded in). Otherwise the content of the
 folded-in part will be that of the cited block.

This very part of the page is an example of \\fold,
whose command is \\fold{fold}{<tt>\\\\fold</tt>}.

For an example of foldable part within a same block, the following

\\fold{}{Title of the part.}{Content of the part once folded in.}

gives
\fold{}{Title of the part.}{Content of the part once folded in.}

Attention. The foldable parts within a same block do not allow
automatic formatting of mathematical formulas, nor execution of
commands.
