diff --git a/extra/sequences/extras/extras-tests.factor b/extra/sequences/extras/extras-tests.factor
index 797a5334f4..c68c26b812 100644
--- a/extra/sequences/extras/extras-tests.factor
+++ b/extra/sequences/extras/extras-tests.factor
@@ -166,3 +166,6 @@ IN: sequences.extras.tests
 { { 1 0 0 1 0 0 0 1 0 0 } }
 [ 1 { 0 3 7 } 10 0 <array> [ set-nths-unsafe ] keep ] unit-test
 
+{ V{ 1 } } [ 1 flatten1 ] unit-test
+{ { 1 2 3 } } [ { 1 2 3 } flatten1 ] unit-test
+{ { 1 2 3 { { 4 } } } } [ { 1 { 2 } { 3 { { 4 } } } } flatten1 ] unit-test
diff --git a/extra/sequences/extras/extras.factor b/extra/sequences/extras/extras.factor
index 15f7f85bd2..e98ba17475 100644
--- a/extra/sequences/extras/extras.factor
+++ b/extra/sequences/extras/extras.factor
@@ -1,6 +1,6 @@
 USING: accessors arrays assocs fry grouping growable kernel
 locals make math math.order math.ranges sequences
-sequences.private sorting splitting ;
+sequences.deep sequences.private sorting splitting ;
 FROM: sequences => change-nth ;
 IN: sequences.extras
 
@@ -365,3 +365,12 @@ PRIVATE>
 
 : set-nths-unsafe ( value indices seq -- )
     swapd '[ [ _ ] dip _ set-nth-unsafe ] each ; inline
+
+: flatten1 ( obj -- seq )
+    [
+        [
+            dup branch? [
+                [ dup branch? [ % ] [ , ] if ] each
+            ] [ , ] if
+        ]
+    ] keep dup branch? [ drop f ] unless make ;