Fix prose

Author: sffc

spec/abstractops.html

@@ -581,7 +581,7 @@ <h1>
     </h1>
     <dl class="header">
       <dt>description</dt>
-      <dd>It validates that the result of GetTemporalUnitValuedOption is <emu-not-ref>covered</emu-not-ref> by the union of _unitGroup_ and _extraValues_.</dd>
+      <dd>It validates that the result of GetTemporalUnitValuedOption is <emu-not-ref>covered</emu-not-ref> by the union of _unitGroup_, _extraValues_, and « ~unset~ ».</dd>
     </dl>
     <emu-alg>
       1. If _value_ is ~unset~, return ~unused~.