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.