diff --git a/web/app/3d/modeler-app.js b/web/app/3d/modeler-app.js
index 1739589c..986a7362 100644
--- a/web/app/3d/modeler-app.js
+++ b/web/app/3d/modeler-app.js
@@ -18,6 +18,7 @@ import '../../css/app3d.less'
 
 import * as BREPPrimitives from '../brep/brep-primitives'
 import * as BREPBool from '../brep/operations/boolean'
+import {BREPValidator} from '../brep/brep-validator'
 import {SceneSolid} from '../brep/viz/scene-solid'
 
 function App() {
@@ -82,6 +83,8 @@ App.prototype.BREPTestImpl = function() {
   const box1 = BREPPrimitives.box(500, 500, 500);
   const box2 = BREPPrimitives.box(500, 500, 500, new Matrix3().translate(250, 250, 250));
 
+  BREPValidator.validateToConsole(box1);
+  
   //box1.faces = [box1.faces[2]];
   //box2.faces = [box2.faces[5]];
   
diff --git a/web/app/brep/brep-builder.js b/web/app/brep/brep-builder.js
index 1a0e3cfd..7b28a622 100644
--- a/web/app/brep/brep-builder.js
+++ b/web/app/brep/brep-builder.js
@@ -17,18 +17,20 @@ export function createPrism(basePoints, height) {
   const lidNormal = normal.multiply(-1);
   const offVector = lidNormal.multiply(height);
   
-  const lidSegments = [];
-  iterateSegments(basePoints.map(p => p.plus(offVector)), (a, b) => lidSegments.push({a, b}));
-  const lidLoop = new Loop();
+  //iterateSegments(basePoints.map(p => new Vertex(p.plus(offVector))), (a, b) => lidSegments.push({a, b}));
+  const lidPoints = basePoints.map(p => p.plus(offVector)).reverse();
+  const lidLoop = createPlaneLoop(lidPoints.map(p => new Vertex(p)));
 
   const shell = new Shell();
 
   const n = baseLoop.halfEdges.length;
   for (let i = 0; i < n; i++) {
+    let lidIdx = n - 2 - i;
+    if (lidIdx == -1) {
+      lidIdx = n - 1;
+    }
     const baseHalfEdge = baseLoop.halfEdges[i];
-    const lidSegment = lidSegments[i];
-    const lidHalfEdge = createHalfEdge(lidLoop, new Vertex(lidSegment.b), new Vertex(lidSegment.a)); 
-    
+    const lidHalfEdge = lidLoop.halfEdges[lidIdx];
     const wallPolygon = [baseHalfEdge.vertexB, baseHalfEdge.vertexA, lidHalfEdge.vertexB, lidHalfEdge.vertexA];
     const wallLoop = createPlaneLoop(wallPolygon);
 
diff --git a/web/app/brep/brep-validator.js b/web/app/brep/brep-validator.js
new file mode 100644
index 00000000..e4a94750
--- /dev/null
+++ b/web/app/brep/brep-validator.js
@@ -0,0 +1,146 @@
+import Vector from '../math/vector'
+import * as math from '../math/math'
+
+export class BREPValidator {
+  
+  constructor() {
+    this.errors = [];
+  }
+  
+  validateShell(shell) {
+    for (let face of shell.faces) {
+      this.validateFace(face);
+    }
+  }
+
+  validateFace(face) {
+    if (face !== face.outerLoop.face) {
+      this.addError(new LoopRefersToWrongFace(face.outerLoop, face));
+    }
+    this.validateLoop(face.outerLoop);
+  }
+
+  validateLoop(loop) {
+    const halfEdges = loop.halfEdges;
+    const n = halfEdges.length;
+    if (n == 0) {
+      return;
+    }
+
+    for (let i = 0; i < n; i ++) {
+      const j = (i + 1) % n;
+      const curr = halfEdges[i];
+      const next = halfEdges[j];
+      if (curr.loop !== loop) {
+        this.addError(new HalfEdgeRefersToWrongLoop(loop, curr));
+      }
+      if (curr.vertexB !== next.vertexA) {
+        this.addError(new VerticesOfHalfEdgeArentConnected(curr, next));
+      }
+      if (curr.next != next) {
+        this.addError(new HalfEdgeNextPointerIncorrect(curr, next));
+      }
+      if (next.prev != curr) {
+        this.addError(new HalfEdgePrevPointerIncorrect(next, curr));
+      }
+      const twin = curr.twin();
+      if (twin.vertexB != curr.vertexA) {
+        this.addError(new TwinStartVertexIncorrect(curr, twin));
+      }
+      if (twin.vertexA != curr.vertexB) {
+        this.addError(new TwinEndVertexIncorrect(curr, twin));
+      }
+    }
+  }
+
+  addError(validationError) {
+    this.errors.push(validationError);
+  }  
+}
+
+BREPValidator.validateToConsole = function(shell) {
+  const brepValidator = new BREPValidator();
+
+  brepValidator.validateShell(shell);
+  for (let brepError of brepValidator.errors) {
+    console.log(brepError.message());
+  }
+};
+
+class VerticesOfHalfEdgeArentConnected {
+  constructor(loop, halfEdge1, halfEdge2) {
+    this.loop = loop;
+    this.halfEdge1 = halfEdge1;
+    this.halfEdge2 = halfEdge2;
+  }
+  
+  message() {
+    return 'starting point of the following half edge should identically the same as ending of a half edge';
+  }
+}
+
+class HalfEdgeRefersToWrongLoop {
+  constructor(loop, face) {
+    this.loop = loop;
+    this.face = face;
+  }
+
+  message() {
+    return 'half edge refers to different loop it belongs to';
+  }
+}
+
+class LoopRefersToWrongFace {
+  constructor(loop, halfEdge) {
+    this.loop = loop;
+    this.halfEdge = halfEdge;
+  }
+
+  message() {
+    return 'loop refers to different face it belongs to';
+  }
+}
+
+class HalfEdgeNextPointerIncorrect {
+  constructor(halfEdge, nextHalfEdge) {
+    this.halfEdge = halfEdge;
+    this.nextHalfEdge = nextHalfEdge;
+  }
+
+  message() {
+    return "half edge's next pointer doesn't refer to real next half edge";
+  }
+}
+
+class HalfEdgePrevPointerIncorrect {
+  constructor(halfEdge, prevHalfEdge) {
+    this.halfEdge = halfEdge;
+    this.prevHalfEdge = prevHalfEdge;
+  }
+
+  message() {
+    return "half edge's prev pointer doesn't refer to prior half edge";
+  }
+}
+
+class TwinStartVertexIncorrect {
+  constructor(halfEdge, twin) {
+    this.halfEdge = halfEdge;
+    this.twin = twin;
+  }
+
+  message() {
+    return "a twin has incorrect start vertex, should be identical to the end vertex of the half edge";
+  }
+}
+
+class TwinEndVertexIncorrect {
+  constructor(halfEdge, twin) {
+    this.halfEdge = halfEdge;
+    this.twin = twin;
+  }
+
+  message() {
+    return "a twin has incorrect end vertex, should be identical to the start vertex of the half edge";
+  }
+}
\ No newline at end of file
diff --git a/web/app/brep/operations/boolean.js b/web/app/brep/operations/boolean.js
index 24d27ab7..6c281ee3 100644
--- a/web/app/brep/operations/boolean.js
+++ b/web/app/brep/operations/boolean.js
@@ -30,6 +30,9 @@ export function union( shell1, shell2 ) {
     //if (shell2.faces.indexOf(face) != -1) {
     //  continue;
     //}
+    if (face.debugName == 'lid') {
+      __DEBUG__.Clear();     
+    }
     const edges = face.outerLoop.halfEdges.concat(faceData.newEdges);
     //edges.forEach(e => __DEBUG__.AddLine(e.vertexA.point, e.vertexB.point));
     while (true) {