diff --git a/web/source/kmwcallback.ts b/web/source/kmwcallback.ts
index 6698cad515..40bf95b7c5 100644
--- a/web/source/kmwcallback.ts
+++ b/web/source/kmwcallback.ts
@@ -22,24 +22,32 @@ type KeyboardStore = PlainKeyboardStore; // | ComplexKeyboardStore;
 type RuleChar = string;
 
 class RuleDeadkey {
+  /** Discriminant field - 'd' for Deadkey.
+   */
+  ['t']: 'd';
+
   /**
-   * Existence of this property indicates the object is a deadkey.  
-   * 
    * Value:  the deadkey's ID.
    */
   ['d']: number; // For 'd'eadkey; also reflects the Deadkey class's 'd' property.
 }
 
 class ContextAny {
+  /** Discriminant field - 'a' for `any()`.
+   */
+  ['t']: 'a';
+
   /**
-   * Existence of this property indicates the object reflects an "any" statement.  
-   * 
    * Value:  the store to search.
    */
   ['a']: KeyboardStore; // For 'a'ny statement.
 }
 
 class RuleIndex {
+  /** Discriminant field - 'i' for `index()`.
+   */
+  ['t']: 'i';
+  
   /**
    * Existence of this property indicates the object reflects an "index" statement.  
    * 
@@ -49,6 +57,10 @@ class RuleIndex {
 }
 
 class ContextEx {
+  /** Discriminant field - 'c' for `context()`.
+   */
+  ['t']: 'c';
+  
   /**
    * Existence of this property indicates that the object reflects a "context" Keyman language statement.
    * 
@@ -57,7 +69,8 @@ class ContextEx {
   ['c']: number; // For 'c'ontext statement.
 }
 
-type ContextEntry = RuleChar | RuleDeadkey | ContextAny | RuleIndex | ContextEx;
+type ContextNonCharEntry = RuleDeadkey | ContextAny | RuleIndex | ContextEx;
+type ContextEntry = RuleChar | ContextNonCharEntry;
 
 /**
  * Cache of context storing and retrieving return values from KC
@@ -414,40 +427,56 @@ class KeyboardInterface {
 
     var mismatch = false;
 
+    var assertNever = function(x: never): never {
+      // Could be accessed by improperly handwritten calls to `fullContextMatch`.
+      throw new Error("Unexpected object in fullContextMatch specification: " + x);
+    }
+
     // Stage two:  time to match against the rule specified.
     for(var i=0; i < rule.length; i++) {
-      if(typeof(rule[i]) == 'string') {
+      if(typeof rule[i] == 'string') {
         var str = rule[i] as string;
         if(str != context[i]) {
           mismatch = true;
           break;
         }
-      } else if(rule[i]['d'] !== undefined) {
-        var deadSpec = rule[i] as RuleDeadkey;
-        // We still need to set a flag here; 
-        if(deadSpec['d'] != context[i]) {
-          mismatch = true;
-        } else {
-          deadContext[i].set();
-        }
-      } else if(rule[i]['a'] !== undefined) {
-        var anySpec = rule[i] as ContextAny;
-        // TODO:  Remove the `string` requirement.
-        if(!this.any(i, context[i] as string, anySpec.a)) {
-          mismatch = true;
-        }
-      } else if(rule[i]['i'] !== undefined) {
-        var indexSpec = rule[i] as RuleIndex;
-        var ch = this._Index(indexSpec.i.s, indexSpec.i.o);
+      } else {
+        // TypeScript needs a cast to this intermediate type to do its discriminated union magic.
+        var r = rule[i] as ContextNonCharEntry;
+        switch(r.t) {
+          case 'd':
+            var deadSpec = r as RuleDeadkey;
+            // We still need to set a flag here; 
+            if(deadSpec['d'] != context[i]) {
+              mismatch = true;
+            } else {
+              deadContext[i].set();
+            }
+            break;
+          case 'a':
+            var anySpec = r as ContextAny;
+            // TODO:  Remove the `string` requirement.
+            if(!this.any(i, context[i] as string, anySpec.a)) {
+              mismatch = true;
+            }
+            break;
+          case 'i':
+            var indexSpec = r as RuleIndex;
+            var ch = this._Index(indexSpec.i.s, indexSpec.i.o);
 
-        if(ch != context[i]) {
-          mismatch = true;
-        }
-      } else if(rule[i]['c'] !== undefined) {
-        var contextSpec = rule[i] as ContextEx;
-        
-        if(context[contextSpec.c - 1] != context[i]) {
-          mismatch = true;
+            if(ch != context[i]) {
+              mismatch = true;
+            }
+            break;
+          case 'c':
+            var contextSpec = r as ContextEx;
+            
+            if(context[contextSpec.c - 1] != context[i]) {
+              mismatch = true;
+            }
+            break;
+          default:
+            assertNever(r);
         }
       }
     }
@@ -820,9 +849,10 @@ class KeyboardInterface {
     if(this._AnyIndices[Pn] < Ps._kmwLength()) {   //I3319
       return Ps._kmwCharAt(this._AnyIndices[Pn]);
     } else {
-      /* Should this really be possible for a compiled keyboard?  
-       * Should we throw an error / output a console.error("")?
+      /* Should not be possible for a compiled keyboard, but may arise 
+       * during the development of handwritten keyboards.
        */
+      console.warn("Unmatched contextual index() statement detected in rule with index " + Pn + "!");
       return "";
     }
   }
@@ -841,6 +871,11 @@ class KeyboardInterface {
     var indexChar = this._Index(Ps, Pn-1);
     if(indexChar) {
       this.output(Pdn,Pelem,indexChar);  //I3319
+    } else {
+      /* Should not be possible for a compiled keyboard, but may arise 
+       * during the development of handwritten keyboards.
+       */
+      console.warn("Unmatched output index() statement detected in rule with index " + Pn + "!");
     }
   }
 
@@ -1034,9 +1069,8 @@ class KeyboardInterface {
    * Description  Clear all matched deadkey flags
    */       
   _DeadkeyResetMatched(): void {                   
-    var _Dk = this._DeadKeys;
-    for(var Li = 0; Li < _Dk.length; Li++) {
-      _Dk[Li].reset();
+    for(let dk of this._DeadKeys) {
+      dk.reset();
     }
   }
 
@@ -1062,10 +1096,9 @@ class KeyboardInterface {
    * Description  Adjust saved positions of deadkeys in context
    */       
   _DeadkeyAdjustPos(Lstart: number, Ldelta: number): void {
-    var _Dk = this._DeadKeys;
-    for(var Li = 0; Li < _Dk.length; Li++) {
-      if(_Dk[Li].p > Lstart) {
-        _Dk[Li].p += Ldelta;
+    for(let dk of this._DeadKeys) {
+      if(dk.p > Lstart) {
+        dk.p += Ldelta;
       }
     }
   }
diff --git a/web/unit_tests/cases/engine.js b/web/unit_tests/cases/engine.js
index efaac55407..2a7d41f2af 100644
--- a/web/unit_tests/cases/engine.js
+++ b/web/unit_tests/cases/engine.js
@@ -44,7 +44,7 @@ function runEngineRuleSet(ruleSet, defaultNoun) {
 var DEADKEY_TEST_1 = {
   id: 1,
   // Match condition for rule
-  rule: [{d: 1}],
+  rule: [{t:'d', d: 1}],
   // Start of context relative to cursor
   n: 1,
   ln: 1,
@@ -82,7 +82,7 @@ var DEADKEY_TEST_1 = {
 var DEADKEY_TEST_2 = {
   id: 2,
   // Match condition for rule
-  rule: ['a', {d: 0}, {d: 1}, 'b'],
+  rule: ['a', {t:'d', d: 0}, {t:'d', d: 1}, 'b'],
   // Start of context relative to cursor
   n: 5,
   ln: 4,
@@ -150,7 +150,7 @@ var DEADKEY_TEST_2 = {
 var DEADKEY_TEST_3 = {
   id: 3,
   // Match condition for rule
-  rule: [{d: 0}, 'a', {d: 0}, {d: 0}, 'b'],
+  rule: [{t:'d', d: 0}, 'a', {t:'d', d: 0}, {t:'d', d: 0}, 'b'],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -238,7 +238,7 @@ var DEADKEY_TEST_3 = {
 var DEADKEY_TEST_4 = {
   id: 4,
   // Match condition for rule
-  rule: ['a', {d: 0}, {d: 0}, 'b', {d: 0}],
+  rule: ['a', {t:'d', d: 0}, {t:'d', d: 0}, 'b', {t:'d', d: 0}],
   // Start of context relative to cursor
   n: 6,
   ln: 5,
@@ -280,7 +280,7 @@ var DEADKEY_TEST_5 = {
 var DEADKEY_TEST_6 = {
   id: 6,
   // Match condition for rule
-  rule: [{d: 1}, {d: 2}, {d: 0}, {d: 1}, {d: 2}],
+  rule: [{t:'d', d: 1}, {t:'d', d: 2}, {t:'d', d: 0}, {t:'d', d: 1}, {t:'d', d: 2}],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -301,7 +301,7 @@ var DEADKEY_TEST_6 = {
 var ANY_CONTEXT_TEST_1 = {
   id: 1,
   // Match condition for rule
-  rule: ['c', "a", "b", {c:3}, {c:2}],
+  rule: ['c', "a", "b", {t:'c', c:3}, {t:'c', c:2}],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -341,7 +341,7 @@ var ANY_CONTEXT_TEST_1 = {
 var ANY_CONTEXT_TEST_2 = {
   id: 2,
   // Match condition for rule
-  rule: ['c', 'a', {a: "bc"}, {c:3}, 'a'],
+  rule: ['c', 'a', {t:'a', a: "bc"}, {t:'c', c:3}, 'a'],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -381,7 +381,7 @@ var ANY_CONTEXT_TEST_2 = {
 var ANY_CONTEXT_TEST_3 = {
   id: 3,
   // Match condition for rule
-  rule: ['c', {a: "ac"}, {a: "bc"}, {c:3}, {c:2}],
+  rule: ['c', {t:'a', a: "ac"}, {t:'a', a: "bc"}, {t:'c', c:3}, {t:'c', c:2}],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -431,7 +431,7 @@ var ANY_CONTEXT_TEST_3 = {
 var ANY_INDEX_TEST_1 = {
   id: 1,
   // Match condition for rule
-  rule: ['c', 'a', {a: "bc"}, {i:{s:"bc", o:2}}, 'a'],
+  rule: ['c', 'a', {t:'a', a: "bc"}, {t:'i', i:{s:"bc", o:2}}, 'a'],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -471,7 +471,7 @@ var ANY_INDEX_TEST_1 = {
 var ANY_INDEX_TEST_2 = {
   id: 2,
   // Match condition for rule
-  rule: ['c', {a:"ab"}, {i: {s:"bc", o:1}}, {i:{s:"bc", o:1}}, {i:{s:"ab", o:1}}],
+  rule: ['c', {t:'a', a:"ab"}, {t:'i', i: {s:"bc", o:1}}, {t:'i', i:{s:"bc", o:1}}, {t:'i', i:{s:"ab", o:1}}],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
@@ -511,7 +511,7 @@ var ANY_INDEX_TEST_2 = {
 var ANY_INDEX_TEST_3 = {
   id: 3,
   // Match condition for rule
-  rule: ['c', {a:"ab"}, {a:"bc"}, {i:{s:"bc", o:2}}, {i:{s:"ab", o:1}}],
+  rule: ['c', {t:'a', a:"ab"}, {t:'a', a:"bc"}, {t:'i', i:{s:"bc", o:2}}, {t:'i', i:{s:"ab", o:1}}],
   // Start of context relative to cursor
   n: 5,
   ln: 5,
diff --git a/web/unit_tests/resources/keyboards/temp_test_deadkeys_hand.js b/web/unit_tests/resources/keyboards/temp_test_deadkeys_hand.js
index 8cd7f90ac5..060985e997 100644
--- a/web/unit_tests/resources/keyboards/temp_test_deadkeys_hand.js
+++ b/web/unit_tests/resources/keyboards/temp_test_deadkeys_hand.js
@@ -39,23 +39,23 @@ function Keyboard_test_deadkeys()
   };
   this.g_main=function(t,e) {
     var k=KeymanWeb,r=0,m=0;
-    if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(7,t,['(','o',')',{d:18},'(','o',')'])) {   // Line 51
+    if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(7,t,['(','o',')',{t:'d',d:18},'(','o',')'])) {   // Line 51
       r=m=1;
       k.KO(6,t,"dk(o)");
     }
-    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(4,t,[{d:18},'(','m',')'])) {   // Line 45
+    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(4,t,[{t:'d',d:18},'(','m',')'])) {   // Line 45
       r=m=1;
       k.KO(3,t,"dk(m)");
     }
-    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(4,t,['(','n',')',{d:18}])) {   // Line 48
+    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(4,t,['(','n',')',{t:'d',d:18}])) {   // Line 48
       r=m=1;
       k.KO(3,t,"dk(n)");
     }
-    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(2,t,[{d:16},{d:17}])) {   // Line 40
+    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(2,t,[{t:'d',d:16},{t:'d',d:17}])) {   // Line 40
       r=m=1;
       k.KO(0,t,"(a)-(s)");
     }
-    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(2,t,[{d:17},{d:16}])) {   // Line 41
+    else if(k.KKM(e, 0x4000, 0xBE)&&k.KFCM(2,t,[{t:'d',d:17},{t:'d',d:16}])) {   // Line 41
       r=m=1;
       k.KO(0,t,"(s)+(a)");
     }
@@ -107,12 +107,12 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KDO(0,t,16);
     }
-    else if(k.KKM(e, 0x4010, 0x42)&&k.KFCM(1, t, [{d:16}])) {   // Line 94
+    else if(k.KKM(e, 0x4010, 0x42)&&k.KFCM(1, t, [{t:'d',d:16}])) {   // Line 94
       r=m=1;
       k.KO(0,t,"ab");
       k.KDO(-1,t,21);
     }
-    else if(k.KKM(e, 0x4010, 0x43)&&k.KFCM(1, t, [{d:22}])) {   // Line 95
+    else if(k.KKM(e, 0x4010, 0x43)&&k.KFCM(1, t, [{t:'d',d:22}])) {   // Line 95
       r=m=1;
       k.KO(0,t,"abc");
     }
@@ -120,7 +120,7 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KO(3,t,"$abc$d");
     }
-    else if(k.KKM(e, 0x4010, 0x45)&&k.KFCM(4,t,['a','b',{d:23},'d'])) {   // Line 96
+    else if(k.KKM(e, 0x4010, 0x45)&&k.KFCM(4,t,['a','b',{t:'d',d:23},'d'])) {   // Line 96
       r=m=1;
       k.KO(3,t,"success");
     }
@@ -130,15 +130,15 @@ function Keyboard_test_deadkeys()
       k.KDO(-1,t,20);
       k.KO(-1,t,"(P)");
     }
-    else if(k.KKM(e, 0x4010, 0x51)&&k.KFCM(4,t,[{d:20},'(','P',')'])) {   // Line 59
+    else if(k.KKM(e, 0x4010, 0x51)&&k.KFCM(4,t,[{t:'d',d:20},'(','P',')'])) {   // Line 59
       r=m=1;
       k.KO(3,t,"(P)(Q)");
     }
-    else if(k.KKM(e, 0x4010, 0x52)&&k.KFCM(8,t,[{d:20},{d:20},'(','P',')','(','Q',')'])) {   // Line 61
+    else if(k.KKM(e, 0x4010, 0x52)&&k.KFCM(8,t,[{t:'d',d:20},{t:'d',d:20},'(','P',')','(','Q',')'])) {   // Line 61
       r=m=1;
       k.KO(6,t,"ERROR");
     }
-    else if(k.KKM(e, 0x4010, 0x52)&&k.KFCM(7,t,[{d:20},'(','P',')','(','Q',')'])) {   // Line 60
+    else if(k.KKM(e, 0x4010, 0x52)&&k.KFCM(7,t,[{t:'d',d:20},'(','P',')','(','Q',')'])) {   // Line 60
       r=m=1;
       k.KO(6,t,"(P)(Q)(R)");
     }
@@ -146,23 +146,23 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KDO(0,t,16);
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{d:2},'d','e','a','d','1'])) {   // Line 84
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{t:'d',d:2},'d','e','a','d','1'])) {   // Line 84
       r=m=1;
       k.KO(5,t,"ERROR");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{d:1},'d','e','a','d','2'])) {   // Line 85
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{t:'d',d:1},'d','e','a','d','2'])) {   // Line 85
       r=m=1;
       k.KO(5,t,"success");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{d:4},'d','e','a','d','3'])) {   // Line 89
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{t:'d',d:4},'d','e','a','d','3'])) {   // Line 89
       r=m=1;
       k.KO(5,t,"success");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{d:3},'d','e','a','d','4'])) {   // Line 90
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(6,t,[{t:'d',d:3},'d','e','a','d','4'])) {   // Line 90
       r=m=1;
       k.KO(5,t,"ERROR");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(4,t,['(','b',')',{d:16}])) {   // Line 24
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(4,t,['(','b',')',{t:'d',d:16}])) {   // Line 24
       r=m=1;
       k.KO(3,t,"(b)+(a)+(b)");
     }
@@ -170,23 +170,23 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KO(3,t,"(b)+(b)");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{d:16}])) {   // Line 19
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{t:'d',d:16}])) {   // Line 19
       r=m=1;
       k.KO(0,t,"(a)(b)");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{d:2}])) {   // Line 82
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{t:'d',d:2}])) {   // Line 82
       r=m=1;
       k.KO(0,t,"dead2");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{d:1}])) {   // Line 83
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{t:'d',d:1}])) {   // Line 83
       r=m=1;
       k.KO(0,t,"dead1");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{d:4}])) {   // Line 87
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{t:'d',d:4}])) {   // Line 87
       r=m=1;
       k.KO(0,t,"dead4");
     }
-    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{d:3}])) {   // Line 88
+    else if(k.KKM(e, 0x4000, 0x42)&&k.KFCM(1, t, [{t:'d',d:3}])) {   // Line 88
       r=m=1;
       k.KO(0,t,"dead3");
     }
@@ -194,7 +194,7 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KO(0,t,"(b)");
     }
-    else if(k.KKM(e, 0x4000, 0x43)&&k.KFCM(1, t, [{d:16}])) {   // Line 27
+    else if(k.KKM(e, 0x4000, 0x43)&&k.KFCM(1, t, [{t:'d',d:16}])) {   // Line 27
       r=m=1;
       k.KO(0,t,"x");
     }
@@ -202,11 +202,11 @@ function Keyboard_test_deadkeys()
       r=m=1;
       k.KO(0,t,"d");
     }
-    else if(k.KKM(e, 0x4000, 0x45)&&k.KFCM(3,t,['f',{d:16},'d'])) {   // Line 35
+    else if(k.KKM(e, 0x4000, 0x45)&&k.KFCM(3,t,['f',{t:'d',d:16},'d'])) {   // Line 35
       r=m=1;
       k.KO(2,t,"(f)-(a)-(d)-(e)");
     }
-    else if(k.KKM(e, 0x4000, 0x45)&&k.KFCM(2,t,[{d:16},'d'])) {   // Line 34
+    else if(k.KKM(e, 0x4000, 0x45)&&k.KFCM(2,t,[{t:'d',d:16},'d'])) {   // Line 34
       r=m=1;
       k.KO(1,t,"(a)(d)(e)");
     }
@@ -236,11 +236,11 @@ function Keyboard_test_deadkeys()
       k.KDO(-1,t,10);
       k.KO(-1,t,"(p)");
     }
-    else if(k.KKM(e, 0x4000, 0x51)&&k.KFCM(4,t,[{d:10},'(','p',')'])) {   // Line 55
+    else if(k.KKM(e, 0x4000, 0x51)&&k.KFCM(4,t,[{t:'d',d:10},'(','p',')'])) {   // Line 55
       r=m=1;
       k.KO(3,t,"(p)(q)");
     }
-    else if(k.KKM(e, 0x4000, 0x52)&&k.KFCM(7,t,[{d:19},'(','p',')','(','q',')'])) {   // Line 56
+    else if(k.KKM(e, 0x4000, 0x52)&&k.KFCM(7,t,[{t:'d',d:19},'(','p',')','(','q',')'])) {   // Line 56
       r=m=1;
       k.KO(6,t,"(p)(q)(r)");
     }
@@ -262,16 +262,16 @@ function Keyboard_test_deadkeys()
       k.KDO(-1,t,23);
       k.KO(-1,t,"d");
     }
-    else if(k.KFCM(3,t,['a','b',{d:21}])) {   // Line 133
+    else if(k.KFCM(3,t,['a','b',{t:'d',d:21}])) {   // Line 133
       m=1;
       k.KDO(2,t,22);
     }
-    else if(k.KFCM(2,t,[{d:2},{d:1}])) {   // Line 130
+    else if(k.KFCM(2,t,[{t:'d',d:2},{t:'d',d:1}])) {   // Line 130
       m=1;
       k.KDO(0,t,1);
       k.KDO(-1,t,2);
     }
-    else if(k.KFCM(2,t,[{d:3},{d:4}])) {   // Line 131
+    else if(k.KFCM(2,t,[{t:'d',d:3},{t:'d',d:4}])) {   // Line 131
       m=1;
       k.KDO(0,t,4);
       k.KDO(-1,t,3);