12.5.4 Spaces
12.5.4.1 Display Spaces
Objects of type display-space are used to describe the desired space between displayed areas.
(display-space? obj)
Returns #t if obj is an object of type display-space, and otherwise returns #f.
(display-space length-spec #!key min: max: conditional?: priority:)
Returns an object of type display-space.  length-spec specifies the nominal size of the space.  min: and max: are length-specs 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 display-spaces 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 display-space 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.

NOTE 51

This allows spaces to disappear at page or column breaks.
12.5.4.2 Inline Spaces
Objects of type inline-space are used to describe the desired space between inline areas.
(inline-space? obj)
Returns #t if obj is an object of type inline-space, and otherwise returns #f.
(inline-space length-spec #!key min: max:)
Returns an object of type inline-space. length-spec specifies the nominal size of the space. min: and max: are length-specs specifying the minimum and maximum size of the space. These both default to the nominal size.