diff --git a/src/mol-model/structure/query/queries/modifiers.ts b/src/mol-model/structure/query/queries/modifiers.ts
index 2a6641adb0e5bb5375dc9471a0f59dcb1a9d2980..5812101ae1b52d63f16d628b19971234e20e2126 100644
--- a/src/mol-model/structure/query/queries/modifiers.ts
+++ b/src/mol-model/structure/query/queries/modifiers.ts
@@ -4,7 +4,7 @@
  * @author David Sehnal <david.sehnal@gmail.com>
  */
 
-import { Segmentation } from 'mol-data/int';
+import { Segmentation, SortedArray } from 'mol-data/int';
 import { Structure, Unit } from '../../structure';
 import { StructureQuery } from '../query';
 import { StructureSelection } from '../selection';
@@ -297,4 +297,86 @@ export function expandProperty(query: StructureQuery, property: QueryFn): Struct
     };
 }
 
-// TODO: unionBy (skip this one?), cluster, includeConnected
\ No newline at end of file
+export interface IncludeConnectedParams {
+    query: StructureQuery,
+    bondTest?: QueryFn<boolean>,
+    layerCount: number,
+    wholeResidues: boolean
+}
+
+export function includeConnected({ query, layerCount, wholeResidues, bondTest }: IncludeConnectedParams): StructureQuery {
+    return ctx => {
+        return 0 as any;
+    }
+}
+
+function defaultBondTest(ctx: QueryContext) {
+    return true;
+}
+
+interface IncludeConnectedCtx {
+    queryCtx: QueryContext,
+    input: Structure,
+    bondTest: QueryFn<boolean>,
+    wholeResidues: boolean
+}
+
+type FrontierSet = UniqueArray<StructureElement.UnitIndex, StructureElement.UnitIndex>
+type Frontier = { unitIds: UniqueArray<number>, elements: Map<number /* unit id */, FrontierSet> }
+
+namespace Frontier {
+    export function has({ elements }: Frontier, unitId: number, element: StructureElement.UnitIndex) {
+        if (!elements.has(unitId)) return false;
+        const xs = elements.get(unitId)!;
+        return xs.keys.has(element);
+    }
+
+    export function create(pivot: Structure, input: Structure) {
+        const unitIds = UniqueArray.create<number>();
+        const elements: Frontier['elements'] = new Map();
+        for (const unit of pivot.units) {
+            if (!Unit.isAtomic(unit)) continue;
+
+            UniqueArray.add(unitIds, unit.id, unit.id);
+            const xs: FrontierSet = UniqueArray.create();
+            elements.set(unit.id, xs);
+
+            const pivotElements = unit.elements;
+            const inputElements = input.unitMap.get(unit.id).elements;
+            for (let i = 0, _i = pivotElements.length; i < _i; i++) {
+                const idx = SortedArray.indexOf(inputElements, pivotElements[i]) as StructureElement.UnitIndex;
+                UniqueArray.add(xs, idx, idx);
+            }
+        }
+
+        return { unitIds, elements };
+    }
+
+    export function addFrontier(target: Frontier, from: Frontier) {
+        for (const unitId of from.unitIds.array) {
+            let xs: FrontierSet;
+            if (target.elements.has(unitId)) {
+                xs = target.elements.get(unitId)!;
+            } else {
+                xs = UniqueArray.create();
+                target.elements.set(unitId, xs);
+                UniqueArray.add(target.unitIds, unitId, unitId);
+            }
+
+            for (const e of from.elements.get(unitId)!.array) {
+                UniqueArray.add(xs, e, e);
+            }
+        }
+        return target;
+    }
+
+    export function includeWholeResidues(structure: Structure, frontier: Frontier) {
+        // ...
+    }
+}
+
+function expandFrontier(ctx: IncludeConnectedCtx, currentFrontier: Frontier, result: Frontier): Frontier {
+    return 0 as any;
+}
+
+// TODO: unionBy (skip this one?), cluster
\ No newline at end of file