[typed-om] Add an 'invert a type' operation.

Author: tabatkins

css-typed-om/Overview.bs

@@ -1805,6 +1805,18 @@ is either null or a [=base type=] other than "percent".
     5. Return |finalType|.
 </div>
 
+<div algorithm>
+    To <dfn export for=CSSNumericValue>invert a type</dfn> |type|,
+    perform the following steps:
+
+    1. Let |result| be a new [=type=]
+        with an initially empty [=ordered map=]
+        and an initially null [=percent hint=]
+    1. [=map/For each=] |unit| → |exponent| of |type|,
+        set |result|[|unit|] to (-1 * |exponent|).
+    3. Return |result|.
+</div>
+
 A [=type=] is said to <dfn export for=CSSNumericValue>match</dfn> a CSS production in some circumstances:
 
 * A [=type=] matches <<length>>