representing a maximum length.
Where <selector> is only needed if you wish to fine-tune value truncation length
by context, <selector> is the context in which the truncation is applied.
The structure of selector is a hyphen separated list of selector kind lists.
A selector kind list is a comma separated list of values of one selector kind.
A selector matches if each selector kind list matches; A selector kind list
matches if one of the values matches.

Below are the relevant selector kinds for truncation.

The case selector kind describes the kind of snippet.  The values are:
	vardecl    -- variable declaration without init
	varinit    -- variable declaration with init
	expression -- expression -- note: {name}==scratch-variable-name
	varvalue   -- variable value expression
	assignment -- assign variable
The action selector kind describes what happened to the snippet.  The values are:
	added     -- snippet has been added
	modified  -- an existing snippet has been modified
	replaced  -- an existing snippet has been replaced with a new snippet
Examples:
	/set trunc mymode 80
	/set truncation mymode 45 expression
	/set truncation mymode 0 vardecl-modified,replaced

Note that subsequent selectors for a field may overwrite some or all of previous used selectors -- last one wins

The form without <length> shows the truncation settings.
When the <mode> is specified only the truncation settings for that mode are shown.
Example:
	/set truncation mymode
shows the truncation settings for the mode mymode