fix first y-array test

src/Struct/ItemString.js

@@ -1,4 +1,4 @@
-import Item from './Item.js'
+import { splitHelper, default as Item } from './Item.js'
 
 export default class ItemString extends Item {
   constructor () {
@@ -13,12 +13,24 @@ export default class ItemString extends Item {
     this._content = decoder.readVarString()
     return missing
   }
-  _toBinary (y, encoder) {
-    super._toBinary(y, encoder)
+  _toBinary (encoder) {
+    super._toBinary(encoder)
     encoder.writeVarString(this._content)
   }
   _logString () {
     let s = super._logString()
     return 'ItemString: ' + s
   }
+  _splitAt (y, diff) {
+    if (diff === 0) {
+      return this
+    } else if (diff >= this._length) {
+      return this._right
+    }
+    let item = new ItemString()
+    item._content = this._content.slice(diff)
+    this._content = this._content.slice(0, diff)
+    splitHelper(y, this, item, diff)
+    return item
+  }
 }