[groff] seeking fixed-width breakable space

[Dave Kemper]

Space characters have two different, independent attributes:

1. They can have a fixed width, or they can stretch for line adjustments.
2. They can be breakable or unbreakable.

This results in four different types of spaces:
 - stretchable breakable (a normal space in groff)
 - stretchable unbreakable ("\~" in groff)
 - fixed-width unbreakable (groff offers a variety, of various widths,
or any arbitrary width using \h)
 - fixed-width breakable (?)

As far as I can determine, groff does not offer a fixed-width,
breakable space.  Am I overlooking something?  If not, is there a way
to trick groff into providing this functionality?