Returns an object of type displayspace. lengthspec specifies the
nominal size of the space. min: and max: are lengthspecs
specifying the minimum and maximum size of the space.
These both default to the nominal size.
priority: is either an integer or the symbol force.
The default is 0.
Higher integers indicate higher priorities.
When two displayspaces are adjacent, then if one has a higher
priority than the other, the minimum, nominal, and maximum values
from the higher priority space shall be used, and the
lower priority space shall be ignored.
If the priorities are equal, but one displayspace has a higher nominal
value than the other, then the minimum, nominal, and maximum values
from the space with the higher nominal value shall be used, and
the other space shall be ignored.
Otherwise, the priorities and nominal values are both equal;
in this case, that nominal value, the lesser of the maximum values,
and the greater of the minimum values shall be used.
A priority of force is considered greater than any other
priority.
However, if both priorities are force, then the
nominal, minimum, and maximum values shall be added together. The
conditional: argument is a boolean; if true, the space shall be
discarded if it starts an area. The default is #t.
 This allows spaces to disappear at page or column breaks. 

12.5.4.2 Inline Spaces
Objects of type inlinespace are used to describe the desired space
between inline areas.
(inlinespace? obj)
Returns #t if obj is an object of type inlinespace, and
otherwise returns #f.
(inlinespace lengthspec #!key min: max:)
Returns an object of type inlinespace.
lengthspec specifies the nominal size of the space.
min: and max: are lengthspecs
specifying the minimum and maximum size of the space.
These both default to the nominal size.