diff --git a/Sources/Incremental/Incremental.swift b/Sources/Incremental/Incremental.swift
index cdeceea..051654b 100644
--- a/Sources/Incremental/Incremental.swift
+++ b/Sources/Incremental/Incremental.swift
@@ -44,16 +44,16 @@ fileprivate struct Register<A> {
 }
 
 final class Observer {
-    let _height: () -> Int
+    let _height: (_ seen: [AnyObject]) -> Int
     let fire: () -> ()
     var cancelled = false
     
-    init(fire: @escaping () -> (), height: @escaping () -> Int) {
+    init(fire: @escaping () -> (), height: @escaping (_ seen: [AnyObject]) -> Int) {
         self.fire = fire
         self._height = height
     }
-    var height: Int {
-        return _height()
+    func height(_ seen: [AnyObject]) -> Int {
+        return _height(seen + [self])
     }
 }
 
@@ -64,7 +64,7 @@ final class Queue {
     
     func enqueue(_ newObservers: [Observer]) {
         observers.append(contentsOf: newObservers)
-        observers.sort { $0.height < $1.height }
+        observers.sort { $0.height([]) < $1.height([]) }
         process()
     }
     
@@ -98,7 +98,7 @@ public class Observable<A> {
         observer(value)
         return observers.add(Observer(fire: {
             observer(self.value)
-        }, height: {
+        }, height: { _ in
             return 0
         }))
     }
@@ -108,15 +108,16 @@ public class Observable<A> {
         observers.remove(token)
     }
     
-    var height: Int {
-        let maxChildHeight = observers.values.map { $0.height }.max()
+    func height(_ seen: [AnyObject]) -> Int {
+        let newSeen = seen + [self]
+        let maxChildHeight = observers.values.filter { child in !newSeen.contains { $0 === child } }.map { $0.height(newSeen) }.max()
         return (maxChildHeight ?? 0) + 1
     }
     
     @discardableResult func addChild<A>(fire: @escaping () -> (), dependent: @escaping () -> Observable<A>) -> Token {
         fire()
         return observers.add(Observer(fire: fire, height: {
-            dependent().height
+            dependent().height($0)
         }))
     }
     
diff --git a/Tests/IncrementalTests/IncrementalTests.swift b/Tests/IncrementalTests/IncrementalTests.swift
new file mode 100644
index 0000000..edb4f99
--- /dev/null
+++ b/Tests/IncrementalTests/IncrementalTests.swift
@@ -0,0 +1,45 @@
+import Foundation
+import XCTest
+import Incremental
+
+func &&(lhs: Observable<Bool>, rhs: Observable<Bool>) -> Observable<Bool> {
+    return lhs.flatMap { x in rhs.map { $0 && x } }
+}
+
+final class IncrementalTests: XCTestCase {
+    func testFlatMapNested() {
+        let o = Observable(0)
+        let o2 = o.flatMap { _ in o }
+        var results: [Int] = []
+        o2.observe { results.append($0) }
+        o.send(1) // eventually crashes here
+        XCTAssert(results == [0, 1])
+    }
+    
+    func testFlatMapMap() {
+        let x = Observable(1)
+        let double = x.flatMap { value in x.map { value + $0 }}
+        var result: [Int] = []
+        let disposable = double.observe { result.append($0) }
+        x.send(2)
+        XCTAssertEqual(result, [2,4])
+    }
+    
+    func test() {
+        let airplaneMode = Observable<Bool>(false)
+        let cellular = Observable<Bool>(true)
+        let wifi = Observable<Bool>(true)
+        
+        let notAirplaneMode = airplaneMode.map { !$0 }
+        
+        let cellularEnabled = notAirplaneMode && cellular
+        let wifiEnabled = notAirplaneMode && wifi
+        let wifiAndCellular = wifiEnabled && cellularEnabled
+        
+        var results: [Bool] = []
+        _ = wifiAndCellular.observe { results.append($0) }
+        airplaneMode.send(true)
+        airplaneMode.send(false)
+        XCTAssertEqual(results, [true,false,true])
+    }
+}