function(i,l){
  var end = this.v$.spanFrom(i+l);
  this.v$=this.v$.measure(0,i).plus(end);
  return this;
}

@@ -8,16 +8,187 @@
 n(i,l)%7B%0A
+  if (i%3Ethis.size)throw AssertionError('StringBuilder.delete index out of bounds');%0A  if (i+l%3Ethis.size)throw AssertionError('StringBuilder.delete length out of bounds');%0A
   var en

validate index and length