4 Helpers
Creates a stretchable snip with the given initial stretchability.
Returns a subclass of 
snip% that takes a single
 initialization argument. The argument provided when instantiating the
 class must be a member of 
possible-labels; the given label
 is displayed by the snip, but the snip is sized to match the longest
 of the labels in 
possible-labels.
In other words, the resulting class helps align multiple GUI elements
t   hat are labeled from a particular set of strings.
An interface for tabbing between embedded 
text%s.
Called when tabbing ahead.
Called when tabbing backward.
Adds the 
tabbable-text<%> interface to an
editor:text% class, where instantiation installs key
bindings to tab ahead and backward
Gives an 
editor-snip% a colored background indicating that
is disabled. The editor is not disabled by the mixin however, and
must be locked separately.
Gives an 
editor<%> a colored background indicating that is
disabled. The editor is not disabled by the mixin however, and must be
locked separately.
Restricts a text to one line by overriding its key bindings to do
nothing on enter.
Gives a 
text% an instantiation argument of a string that is
displayed in the 
text% initially in grey; the text
disappears when the text gets focus. This technique is useful for
labeling texts without needing to take up space.
Creates an instance with the given initial content, color, and
behvior for when to clear the text.
Clears the cue text, if it’s still present.