diff --git a/libs/microbit/core.cpp b/libs/microbit/core.cpp
index c78fca9d..80e2722f 100644
--- a/libs/microbit/core.cpp
+++ b/libs/microbit/core.cpp
@@ -78,6 +78,11 @@ namespace String {
     {
       return ManagedString((char)code).leakData();
     }
+
+    //%
+    int toNumber(StringData *s) {
+      return atoi(s->data);
+    }
 }
 
 namespace NumberMethods {
@@ -137,3 +142,92 @@ namespace Math {
       return ::sqrt(x);
     }
 }
+
+namespace ArrayImpl {
+    //%
+    RefCollection *mk(uint32_t flags)
+    {
+      RefCollection *r = new RefCollection(flags);
+      return r;
+    }
+
+    //%
+    int length(RefCollection *c) { return c->data.size(); }
+
+    //%
+    void push(RefCollection *c, uint32_t x) {
+      if (c->flags & 1) incr(x);
+      c->data.push_back(x);
+    }
+
+    inline bool in_range(RefCollection *c, int x) {
+      return (0 <= x && x < (int)c->data.size());
+    }
+
+    //%
+    uint32_t getAt(RefCollection *c, int x) {
+      if (in_range(c, x)) {
+        uint32_t tmp = c->data.at(x);
+        if (c->flags & 1) incr(tmp);
+        return tmp;
+      }
+      else {
+        error(ERR_OUT_OF_BOUNDS);
+        return 0;
+      }
+    }
+
+    //%
+    void removeAt(RefCollection *c, int x) {
+      if (!in_range(c, x))
+        return;
+
+      if (c->flags & 1) decr(c->data.at(x));
+      c->data.erase(c->data.begin()+x);
+    }
+
+    //%
+    void setAt(RefCollection *c, int x, uint32_t y) {
+      if (!in_range(c, x))
+        return;
+
+      if (c->flags & 1) {
+        decr(c->data.at(x));
+        incr(y);
+      }
+      c->data.at(x) = y;
+    }
+
+    //%
+    int indexOf(RefCollection *c, uint32_t x, int start) {
+      if (!in_range(c, start))
+        return -1;
+
+      if (c->flags & 2) {
+        StringData *xx = (StringData*)x;
+        for (uint32_t i = start; i < c->data.size(); ++i) {
+          StringData *ee = (StringData*)c->data.at(i);
+          if (xx->len == ee->len && memcmp(xx->data, ee->data, xx->len) == 0)
+            return (int)i;
+        }
+      } else {
+        for (uint32_t i = start; i < c->data.size(); ++i)
+          if (c->data.at(i) == x)
+            return (int)i;
+      }
+
+      return -1;
+    }
+
+    //%
+    int removeElement(RefCollection *c, uint32_t x) {
+      int idx = indexOf(c, x, 0);
+      if (idx >= 0) {
+        removeAt(c, idx);
+        return 1;
+      }
+
+      return 0;
+    }
+}
+
diff --git a/libs/microbit/core.d.ts b/libs/microbit/core.d.ts
index f1990c39..07a1492f 100644
--- a/libs/microbit/core.d.ts
+++ b/libs/microbit/core.d.ts
@@ -4,14 +4,14 @@ interface Array<T> {
     /**
       * Gets or sets the length of the array. This is a number one higher than the highest element defined in an array.
       */
-    //% shim=collection::count
+    //% shim=ArrayImpl::length
     length: number;
 
     /**
       * Appends new elements to an array.
       * @param items New elements of the Array.
       */
-    //% shim=collection::add
+    //% shim=ArrayImpl::push
     push(item: T): void;
     
     /**
@@ -41,11 +41,11 @@ interface Array<T> {
     slice(start: number, end: number): T[];
 
     /** Removes the first occurence of an object. Returns true if removed. */
-    //% shim=collection::remove
+    //% shim=ArrayImpl::removeElement
     removeElement(element:T) : boolean;
     
     /** Removes the object at position index. */
-    //% shim=collection::remove_at
+    //% shim=ArrayImpl::removeAt
     removeAt(idx:number) : void;
     
     
@@ -69,7 +69,7 @@ interface Array<T> {
       * @param searchElement The value to locate in the array.
       * @param fromIndex The array index at which to begin the search. If fromIndex is omitted, the search starts at index 0.
       */
-    //% shim=collection::index_of
+    //% shim=ArrayImpl::indexOf
     indexOf(searchElement: T, fromIndex?: number): number;
 
 
@@ -86,7 +86,7 @@ interface String {
   * Converts A string to an integer.
   * @param s A string to convert into a number.
   */
-  //% shim=string::to_number
+  //% shim=String::toNumber
 declare function parseInt(s: string): number;
 
 interface Object {}
@@ -94,27 +94,3 @@ interface Function {}
 interface IArguments {}
 interface RegExp {}
 
-interface Boolean {
-    /**
-      * Returns a string representation of an object.
-      */
-    //% shim=boolean::to_string
-    toString(): string;
-}
-
-declare namespace String {
-    /**
-      * Make a string from the given ASCII character code. 
-      */
-    //% shim=number::to_character
-    export function fromCharCode(code:number): string;
-}
-
-interface Number {
-    /**
-      * Returns a string representation of an object.
-      */
-    //% shim=number::to_string
-    toString(): string;
-}
-
diff --git a/libs/microbit/shims.d.ts b/libs/microbit/shims.d.ts
index fe0355f1..52adeccd 100644
--- a/libs/microbit/shims.d.ts
+++ b/libs/microbit/shims.d.ts
@@ -32,7 +32,7 @@ declare interface String {
 
     /** Returns the length of a String object. */
     //% property shim=StringMethods::length
-    length(): number;
+    length: number;
 }