D语言前进范围

import std.array; 
import std.stdio; 
import std.string; 
import std.range;

struct FibonacciSeries { 
   int first = 0; 
   int second = 1; 
   enum empty = false;   //  infinite range  

   @property int front() const { 
      return first; 
   } 
   void popFront() { 
      int third = first + second; 
      first = second; 
      second = third; 
   }
   @property FibonacciSeries save() const { 
      return this; 
   } 
}

void report(T)(const dchar[] title, const ref T range) {
   writefln("%s: %s", title, range.take(5)); 
} 

void main() { 
   auto range = FibonacciSeries(); 
   report("Original range", range);

   range.popFrontN(2); 
   report("After removing two elements", range); 

   auto theCopy = range.save; 
   report("The copy", theCopy);

   range.popFrontN(3); 
   report("After removing three more elements", range); 
   report("The copy", theCopy); 
}