diff --git a/src/_impl/_bijection_offset_impl.scad b/src/_impl/_bijection_offset_impl.scad
new file mode 100644
index 00000000..1a5043cb
--- /dev/null
+++ b/src/_impl/_bijection_offset_impl.scad
@@ -0,0 +1,56 @@
+use <__comm__/__lines_from.scad>;
+use <__comm__/__line_intersection.scad>;
+    
+function _bijection_inward_edge_normal(edge) =  
+    let(
+        pt1 = edge[0],
+        pt2 = edge[1],
+        dx = pt2[0] - pt1[0],
+        dy = pt2[1] - pt1[1],
+        edge_leng = norm([dx, dy])
+    )
+    [-dy / edge_leng, dx / edge_leng];
+
+function _bijection_outward_edge_normal(edge) = -1 * _bijection_inward_edge_normal(edge);
+
+function _bijection_offset_edge(edge, dx, dy) = 
+    let(
+        pt1 = edge[0],
+        pt2 = edge[1],
+        dxy = [dx, dy]
+    )
+    [pt1 + dxy, pt2 + dxy];
+    
+function _bijection__bijection_offset_edges(edges, d) = 
+    [ 
+        for(edge = edges)
+        let(
+            ow_normal = _bijection_outward_edge_normal(edge),
+            dx = ow_normal[0] * d,
+            dy = ow_normal[1] * d
+        )
+        _bijection_offset_edge(edge, dx, dy)
+    ];
+
+function _bijection_offset_impl(pts, d, epsilon) = 
+    let(
+        es = __lines_from(pts, true), 
+        offset_es = _bijection__bijection_offset_edges(es, d),
+        leng = len(offset_es),
+        leng_minus_one = leng - 1,
+        last_p = __line_intersection(offset_es[leng_minus_one], offset_es[0], epsilon)
+    )
+    concat(
+        last_p != [] && last_p == last_p ? [last_p] : [],
+        [
+            for(i = 0; i < leng_minus_one; i = i + 1)
+            let(
+                this_edge = offset_es[i],
+                next_edge = offset_es[i + 1],
+                p = __line_intersection(this_edge, next_edge, epsilon)
+            )
+            // p == p to avoid [nan, nan], because [nan, nan] != [nan, nan]
+            if(p != [] && p == p) p
+        ]
+    );
+    
\ No newline at end of file
diff --git a/src/bijection_offset.scad b/src/bijection_offset.scad
index b3e885fd..84d019c7 100644
--- a/src/bijection_offset.scad
+++ b/src/bijection_offset.scad
@@ -8,59 +8,8 @@
 *
 **/
 
-include <__comm__/__lines_from.scad>;
-include <__comm__/__line_intersection.scad>;
+use <_impl/_bijection_offset_impl.scad>;
     
-function _bijection_inward_edge_normal(edge) =  
-    let(
-        pt1 = edge[0],
-        pt2 = edge[1],
-        dx = pt2[0] - pt1[0],
-        dy = pt2[1] - pt1[1],
-        edge_leng = norm([dx, dy])
-    )
-    [-dy / edge_leng, dx / edge_leng];
-
-function _bijection_outward_edge_normal(edge) = -1 * _bijection_inward_edge_normal(edge);
-
-function _bijection_offset_edge(edge, dx, dy) = 
-    let(
-        pt1 = edge[0],
-        pt2 = edge[1],
-        dxy = [dx, dy]
-    )
-    [pt1 + dxy, pt2 + dxy];
-    
-function _bijection__bijection_offset_edges(edges, d) = 
-    [ 
-        for(edge = edges)
-        let(
-            ow_normal = _bijection_outward_edge_normal(edge),
-            dx = ow_normal[0] * d,
-            dy = ow_normal[1] * d
-        )
-        _bijection_offset_edge(edge, dx, dy)
-    ];
-
 function bijection_offset(pts, d, epsilon = 0.0001) = 
-    let(
-        es = __lines_from(pts, true), 
-        offset_es = _bijection__bijection_offset_edges(es, d),
-        leng = len(offset_es),
-        leng_minus_one = leng - 1,
-        last_p = __line_intersection(offset_es[leng_minus_one], offset_es[0], epsilon)
-    )
-    concat(
-        last_p != [] && last_p == last_p ? [last_p] : [],
-        [
-            for(i = 0; i < leng_minus_one; i = i + 1)
-            let(
-                this_edge = offset_es[i],
-                next_edge = offset_es[i + 1],
-                p = __line_intersection(this_edge, next_edge, epsilon)
-            )
-            // p == p to avoid [nan, nan], because [nan, nan] != [nan, nan]
-            if(p != [] && p == p) p
-        ]
-    );
+    _bijection_offset_impl(pts, d, epsilon);
     
\ No newline at end of file