diff --git a/main.py b/main.py
index 74ea579..0e828e6 100644
--- a/main.py
+++ b/main.py
@@ -56,3 +56,5 @@ with TimeSample('    full'):
     sys1 = sync(sys1.call(eproxy(77)))
     sys1 = sync(sys1.call(eproxy(194)))
     print(sys1)
+with TimeSample('    full'):
+    print(with_common_interface(synced(readfile('src/sys4'))))
diff --git a/src/@.bu4 b/src/@.bu4
index 95c0d8b..f4847fa 100644
--- a/src/@.bu4
+++ b/src/@.bu4
@@ -4,4 +4,5 @@
 @convert/@
 @io/@
 @serialization/@
+@collections/@
 @numbers
diff --git a/src/collections/@.bu4 b/src/collections/@.bu4
new file mode 100644
index 0000000..b398509
--- /dev/null
+++ b/src/collections/@.bu4
@@ -0,0 +1 @@
+@deque
diff --git a/src/collections/deque.bu4 b/src/collections/deque.bu4
new file mode 100644
index 0000000..89b1191
--- /dev/null
+++ b/src/collections/deque.bu4
@@ -0,0 +1,96 @@
+{        (g) /?     /?           /?     /?           /[1][g] | dequeEmpty  }
+{ (item) (g) /[item]/[dequeEmpty]/[item]/[dequeEmpty]/[0][g] | dequeSingle }
+
+{
+    (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+        [lPopped]
+|
+    dequeLPopped
+}
+
+{
+    (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+        [lItem]
+|
+    dequeLItem
+}
+
+{
+    (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+        [rPopped]
+|
+    dequeRPopped
+}
+
+{
+    (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+        [rItem]
+|
+    dequeRItem
+}
+
+{
+    /(dequeLPush)
+        (deque)(item)
+            /(isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+                /
+                    (g) /[rItem]/</[item]/[rPopped][dequeLPush]>/[item]/[deque]/[0] [g]
+                /
+                    /[item][dequeSingle]
+                [isEmpty]
+            [deque]
+    [YC]
+|
+    dequeLPush
+}
+
+{
+    /(dequeRPush)
+        (deque)(item)
+            /(isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+                /
+                    (g) /[item]/[deque]/[lItem]/</[item]/[lPopped][dequeRPush]>/[0] [g]
+                /
+                    /[item][dequeSingle]
+                [isEmpty]
+            [deque]
+    [YC]
+|
+    dequeRPush
+}
+
+{
+    (_0)(_1)(_add)
+        /(dequeLength)
+            (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+                /
+                    /
+                        /[dequeLength][lPopped]
+                    /
+                        [_1]
+                    [_add]
+                /
+                    [_0]
+                [isEmpty]
+        [YC]
+|
+    dequeLLength
+}
+
+{
+    (_0)(_1)(_add)
+        /(dequeLength)
+            (isEmpty)(lPopped)(lItem)(rPopped)(rItem)
+                /
+                    /
+                        /[dequeLength][rPopped]
+                    /
+                        [_1]
+                    [_add]
+                /
+                    [_0]
+                [isEmpty]
+        [YC]
+|
+    dequeRLength
+}
diff --git a/src/sys4.bu4 b/src/sys4.bu4
new file mode 100644
index 0000000..a15973f
--- /dev/null
+++ b/src/sys4.bu4
@@ -0,0 +1,12 @@
+@@
+
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+@ci
+
+    { /[b_add]/[b1]/[b0][dequeLLength]                   | dequeLength }
+    { /[b!]//[pair]/[dequeEmpty][dequeLPush][dequeLPush] | deque       }
+
+    /
+        /[dequeLength][deque]
+    [b_to_nx]
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""