Implement [, I, and D

Added ability to split elements off the current stack into a new one,
and traverse between stacks.

starfish.js

@@ -314,16 +314,26 @@ class CodeBox {
                 case "l":
                     this.stacks[this.curr_stack].PushLength();
                     break;
-                // case "[":
-                //     break;
+                case "[": {
+                    this.SpliceStack(this.stacks[this.curr_stack].Pop());
+                    break;
+                }
                 // case "]":
                 //     break;
-                // case "I":
-                //     this.curr_stack++;
-                //     break;
-                // case "D":
-                //     this.curr_stack--;
-                //     break;
+                case "I": {
+                    this.curr_stack++;
+                    if (this.curr_stack >= this.stacks.length) {
+                        throw new RangeError("curr_stack value out of bounds");
+                    }
+                    break;
+                }
+                case "D": {
+                    this.curr_stack--;
+                    if (this.curr_stack < 0) {
+                        throw new RangeError("curr_stack value out of bounds");
+                    }
+                    break;
+                }
                 // Output
                 case "n": 
                     output = this.stacks[this.curr_stack].Pop();
@@ -518,6 +528,37 @@ class CodeBox {
         this.curr_direction = Object.values(this.directions)[Math.floor(Math.random() * 4)];
     }
 
+    /**
+     * Takes X number of elements out of a stack and into a new stack
+     *
+     * This action creates a new stack, and places it on top of the one it was created from.
+     * So, if you have three stacks, A, B, and C, and you splice a stack off of stack B,
+     * the new order will be: A, B,  D, and C.
+     *
+     * @see {@link https://esolangs.org/wiki/Fish#Stacks ><> Documentation}
+     *
+     * @param {int} spliceCount The number of elements to pop into a new stack
+     */
+    SpliceStack(spliceCount) {
+        const stackCount = this.stacks[this.curr_stack].stack.length;
+
+        if (spliceCount > stackCount) {
+            throw new RangeError(`Cannot remove ${spliceCount} elements from a stack of only ${stackCount} elements`);
+        }
+
+        const newStack = new Stack(this.stacks[this.curr_stack].stack.splice(stackCount - spliceCount, spliceCount));
+
+        // We're at the top of the stacks stack, so we can use .push
+        if (this.curr_stack == this.stacks.length - 1) {
+            this.stacks.push(newStack);
+        }
+        else {
+            this.stacks.splice(this.curr_stack + 1, 0, newStack);
+        }
+
+        this.curr_stack++;
+    }
+
     /**
      * Implement `
      *