diff --git a/basis/compiler/cfg/alias-analysis/alias-analysis.factor b/basis/compiler/cfg/alias-analysis/alias-analysis.factor
index 23e46f169b..24433ad594 100644
--- a/basis/compiler/cfg/alias-analysis/alias-analysis.factor
+++ b/basis/compiler/cfg/alias-analysis/alias-analysis.factor
@@ -298,7 +298,7 @@ SYMBOL: live-stores
     histories get
     values [
         values [ [ store? ] filter [ insn#>> ] map ] map concat
-    ] map concat HS{ } set-like
+    ] map concat fast-set
     live-stores set ;
 
 GENERIC: eliminate-dead-stores* ( insn -- insn' )
diff --git a/basis/vocabs/hierarchy/hierarchy.factor b/basis/vocabs/hierarchy/hierarchy.factor
index b840b5ab9d..986091a543 100644
--- a/basis/vocabs/hierarchy/hierarchy.factor
+++ b/basis/vocabs/hierarchy/hierarchy.factor
@@ -65,8 +65,8 @@ PRIVATE>
     #! Hack.
     [ vocab-prefix? ] partition
     [
-        [ vocab-name ] map unique
-        '[ name>> _ key? not ] filter
+        [ vocab-name ] map fast-set
+        '[ name>> _ in? not ] filter
         convert-prefixes
     ] keep
     append ;