diff --git a/src/_impl/_trim_shape_impl.scad b/src/_impl/_trim_shape_impl.scad
new file mode 100644
index 00000000..1f614fa7
--- /dev/null
+++ b/src/_impl/_trim_shape_impl.scad
@@ -0,0 +1,48 @@
+use <__comm__/__line_intersection.scad>;
+use <__comm__/__in_line.scad>;
+use <__comm__/__lines_from.scad>;
+
+function _trim_shape_any_intersection_sub(lines, line, lines_leng, i, epsilon) =
+    let(
+        p = __line_intersection(lines[i], line, epsilon)
+    )
+    (p != [] && __in_line(line, p, epsilon) && __in_line(lines[i], p, epsilon)) ? [i, p] : _trim_shape_any_intersection(lines, line, lines_leng, i + 1, epsilon);
+
+// return [idx, [x, y]] or []
+function _trim_shape_any_intersection(lines, line, lines_leng, i, epsilon) =
+    i == lines_leng ? [] : _trim_shape_any_intersection_sub(lines, line, lines_leng, i, epsilon);
+
+function _trim_sub(lines, leng, epsilon) = 
+    let(
+        current_line = lines[0],
+        next_line = lines[1],
+        lines_from_next = [for(j = 1; j < leng; j = j + 1) lines[j]],
+        lines_from_next2 = [for(j = 2; j < leng; j = j + 1) lines[j]],
+        current_p = current_line[0],
+        leng_lines_from_next2 = len(lines_from_next2),
+        inter_p = _trim_shape_any_intersection(lines_from_next2, current_line, leng_lines_from_next2, 0, epsilon)
+    )
+    // no intersecting pt, collect current_p and trim remain lines
+    inter_p == [] ? (concat([current_p], _trim_shape_trim_lines(lines_from_next, epsilon))) : (
+        // collect current_p, intersecting pt and the last pt
+        (leng == 3 || (inter_p[0] == (leng_lines_from_next2 - 1))) ? [current_p, inter_p[1], lines[leng - 1]] : (
+            // collect current_p, intersecting pt and trim remain lines
+            concat([current_p, inter_p[1]], 
+                _trim_shape_trim_lines([for(i = inter_p[0] + 1; i < leng_lines_from_next2; i = i + 1) lines_from_next2[i]], epsilon)
+            )
+        )
+    );
+    
+function _trim_shape_trim_lines(lines, epsilon) = 
+    let(leng = len(lines))
+    leng > 2 ? _trim_sub(lines, leng, epsilon) : _trim_shape_collect_pts_from(lines, leng);
+
+function _trim_shape_collect_pts_from(lines, leng) = 
+    concat([for(line = lines) line[0]], [lines[leng - 1][1]]);
+
+function _trim_shape_impl(shape_pts, from, to, epsilon) = 
+    let(
+        pts = [for(i = from; i <= to; i = i + 1) shape_pts[i]],
+        trimmed = _trim_shape_trim_lines(__lines_from(pts), epsilon)
+    )
+    len(shape_pts) == len(trimmed) ? trimmed : _trim_shape_impl(trimmed, 0, len(trimmed) - 1, epsilon);
diff --git a/src/trim_shape.scad b/src/trim_shape.scad
index c436abc0..db866054 100644
--- a/src/trim_shape.scad
+++ b/src/trim_shape.scad
@@ -8,52 +8,7 @@
 *
 **/
 
-include <__comm__/__to3d.scad>;
-include <__comm__/__line_intersection.scad>;
-include <__comm__/__in_line.scad>;
-include <__comm__/__lines_from.scad>;
-
-function _trim_shape_any_intersection_sub(lines, line, lines_leng, i, epsilon) =
-    let(
-        p = __line_intersection(lines[i], line, epsilon)
-    )
-    (p != [] && __in_line(line, p, epsilon) && __in_line(lines[i], p, epsilon)) ? [i, p] : _trim_shape_any_intersection(lines, line, lines_leng, i + 1, epsilon);
-
-// return [idx, [x, y]] or []
-function _trim_shape_any_intersection(lines, line, lines_leng, i, epsilon) =
-    i == lines_leng ? [] : _trim_shape_any_intersection_sub(lines, line, lines_leng, i, epsilon);
-
-function _trim_sub(lines, leng, epsilon) = 
-    let(
-        current_line = lines[0],
-        next_line = lines[1],
-        lines_from_next = [for(j = 1; j < leng; j = j + 1) lines[j]],
-        lines_from_next2 = [for(j = 2; j < leng; j = j + 1) lines[j]],
-        current_p = current_line[0],
-        leng_lines_from_next2 = len(lines_from_next2),
-        inter_p = _trim_shape_any_intersection(lines_from_next2, current_line, leng_lines_from_next2, 0, epsilon)
-    )
-    // no intersecting pt, collect current_p and trim remain lines
-    inter_p == [] ? (concat([current_p], _trim_shape_trim_lines(lines_from_next, epsilon))) : (
-        // collect current_p, intersecting pt and the last pt
-        (leng == 3 || (inter_p[0] == (leng_lines_from_next2 - 1))) ? [current_p, inter_p[1], lines[leng - 1]] : (
-            // collect current_p, intersecting pt and trim remain lines
-            concat([current_p, inter_p[1]], 
-                _trim_shape_trim_lines([for(i = inter_p[0] + 1; i < leng_lines_from_next2; i = i + 1) lines_from_next2[i]], epsilon)
-            )
-        )
-    );
-    
-function _trim_shape_trim_lines(lines, epsilon) = 
-    let(leng = len(lines))
-    leng > 2 ? _trim_sub(lines, leng, epsilon) : _trim_shape_collect_pts_from(lines, leng);
-
-function _trim_shape_collect_pts_from(lines, leng) = 
-    concat([for(line = lines) line[0]], [lines[leng - 1][1]]);
+use <_impl/_trim_shape_impl.scad>;
 
 function trim_shape(shape_pts, from, to, epsilon = 0.0001) = 
-    let(
-        pts = [for(i = from; i <= to; i = i + 1) shape_pts[i]],
-        trimmed = _trim_shape_trim_lines(__lines_from(pts), epsilon)
-    )
-    len(shape_pts) == len(trimmed) ? trimmed : trim_shape(trimmed, 0, len(trimmed) - 1, epsilon);
+    _trim_shape_impl(shape_pts, from, to, epsilon);
diff --git a/test/test_trim_shape.scad b/test/test_trim_shape.scad
index 185cb1f1..bf867c82 100644
--- a/test/test_trim_shape.scad
+++ b/test/test_trim_shape.scad
@@ -1,7 +1,7 @@
-include <unittest.scad>;    
-include <trim_shape.scad>;
-include <shape_taiwan.scad>;
-include <bijection_offset.scad>;
+use <unittest.scad>;    
+use <trim_shape.scad>;
+use <shape_taiwan.scad>;
+use <bijection_offset.scad>;
 
 module test_trim_shape() {
     echo("==== test_trim_shape ====");