12.5.2 Length Specification
An object of type length-spec specifies a length as a linear combination of other lengths that may not be currently known. Whenever a value of type length-spec is required, a length (a quantity of dimension 1) may always be used.
(+ length-spec )
(- length-spec )
(* length-spec x)
(* x length-spec)
(/ length-spec x)
(/ x length-spec)
These procedures behave in the same way as their counterparts on quantities, except that they shall return a length-spec if any of their arguments is a length-spec (as opposed to just a length).
This procedure shall be used only in the evaluation of an expression specifying a value for a characteristic.  The value flow object shall be a displayed flow object.  It returns a length-spec specifying the display-size of the value flow object.