add if-at, when-at, and unless-at

it's like if-empty

extra/assocs/lib/lib.factor

@@ -37,3 +37,10 @@ IN: assocs.lib
     H{ } clone [
         swap [ change-at ] 2curry assoc-each
     ] keep ; inline
+
+: if-at ( obj assoc quot1 quot2 -- )
+    [ dupd at* [ not -rot ? ] keep ] 2dip if ; inline
+
+: when-at ( obj assoc quot -- ) [ ] if-at ; inline
+
+: unless-at ( obj assoc quot -- ) [ ] swap if-at ; inline