diff --git a/core/compiler/units/units.factor b/core/compiler/units/units.factor
index 72ca0f54e6..b74f96a24e 100644
--- a/core/compiler/units/units.factor
+++ b/core/compiler/units/units.factor
@@ -18,10 +18,10 @@ TUPLE: redefine-error def ;
 
 <PRIVATE
 
-: add-once ( key assoc -- )
-    2dup key? [ over redefine-error ] when conjoin ;
+: add-once ( key set -- )
+    2dup in? [ over redefine-error ] when adjoin ;
 
-: (remember-definition) ( definition loc assoc -- )
+: (remember-definition) ( definition loc set -- )
     [ over set-where ] dip add-once ;
 
 PRIVATE>
@@ -30,16 +30,16 @@ PRIVATE>
     new-definitions get first (remember-definition) ;
 
 : fake-definition ( definition -- )
-    old-definitions get [ delete-at ] with each ;
+    old-definitions get [ delete ] with each ;
 
 : remember-class ( class loc -- )
-    [ dup new-definitions get first key? [ dup redefine-error ] when ] dip
+    [ dup new-definitions get first in? [ dup redefine-error ] when ] dip
     new-definitions get second (remember-definition) ;
 
 : forward-reference? ( word -- ? )
-    dup old-definitions get assoc-stack
-    [ new-definitions get assoc-stack not ]
-    [ drop f ] if ;
+    dup old-definitions get [ in? ] with any? [
+        new-definitions get [ in? ] with any? not
+    ] [ drop f ] if ;
 
 SYMBOL: compiler-impl
 
@@ -81,7 +81,7 @@ M: f process-forgotten-words drop ;
 : without-optimizer ( quot -- )
     [ f compiler-impl ] dip with-variable ; inline
 
-: <definitions> ( -- pair ) { H{ } H{ } } [ clone ] map ;
+: <definitions> ( -- pair ) { HS{ } HS{ } } [ clone ] map ;
 
 SYMBOL: definition-observers
 
@@ -122,8 +122,8 @@ M: object always-bump-effect-counter? drop f ;
 : updated-definitions ( -- set )
     HS{ } clone
     forgotten-definitions get union!
-    new-definitions get first keys over adjoin-all
-    new-definitions get second keys over adjoin-all
+    new-definitions get first union!
+    new-definitions get second union!
     changed-definitions get union!
     maybe-changed get union!
     dup changed-vocabs over adjoin-all ;
diff --git a/core/parser/parser.factor b/core/parser/parser.factor
index 8eb7498d50..9444b117b3 100644
--- a/core/parser/parser.factor
+++ b/core/parser/parser.factor
@@ -5,6 +5,7 @@ compiler.units continuations definitions effects io
 io.encodings.utf8 io.files kernel lexer math.parser namespaces
 parser.notes quotations sequences sets slots source-files
 vectors vocabs vocabs.parser words words.symbol ;
+FROM: sets => members ;
 IN: parser
 
 : location ( -- loc )
@@ -162,8 +163,8 @@ print-use-hook [ [ ] ] initialize
 : parsing-file ( file -- )
     parser-quiet? get [ drop ] [ "Loading " write print flush ] if ;
 
-: filter-moved ( assoc1 assoc2 -- seq )
-    swap assoc-diff keys [
+: filter-moved ( set1 set2 -- seq )
+    swap diff members [
         {
             { [ dup where dup [ first ] when file get path>> = not ] [ f ] }
             { [ dup reader-method? ] [ f ] }
@@ -172,11 +173,11 @@ print-use-hook [ [ ] ] initialize
         } cond nip
     ] filter ;
 
-: removed-definitions ( -- assoc1 assoc2 )
+: removed-definitions ( -- set1 set2 )
     new-definitions old-definitions
-    [ get first2 assoc-union ] bi@ ;
+    [ get first2 union ] bi@ ;
 
-: removed-classes ( -- assoc1 assoc2 )
+: removed-classes ( -- set1 set2 )
     new-definitions old-definitions
     [ get second ] bi@ ;
 
diff --git a/core/source-files/source-files.factor b/core/source-files/source-files.factor
index aa040bdfcd..46c0dda024 100644
--- a/core/source-files/source-files.factor
+++ b/core/source-files/source-files.factor
@@ -2,8 +2,9 @@
 ! See http://factorcode.org/license.txt for BSD license.
 USING: accessors arrays assocs checksums checksums.crc32
 compiler.units continuations definitions io.encodings.utf8
-io.files io.pathnames kernel namespaces sequences
+io.files io.pathnames kernel namespaces sequences sets
 source-files.errors strings words ;
+FROM: namespaces => set ;
 IN: source-files
 
 SYMBOL: source-files
@@ -47,14 +48,14 @@ M: pathname where string>> 1 2array ;
 
 : forget-source ( path -- )
     source-files get delete-at*
-    [ definitions>> [ keys forget-all ] each ] [ drop ] if ;
+    [ definitions>> [ members forget-all ] each ] [ drop ] if ;
 
 M: pathname forget*
     string>> forget-source ;
 
 : rollback-source-file ( file -- )
     [
-        new-definitions get [ assoc-union ] 2map
+        new-definitions get [ union ] 2map
     ] change-definitions drop ;
 
 SYMBOL: file