Debug help - Ranges - Programming in D page 598

[Brother Bill]

If line 9 of the program is commented out, it runs fine.
Otherwise lots of error messages which don't pan out for me.
I tried changing Negative.front to be const, which didn't help.
I am not quite sure what the error messages are telling me.

Thank you for your assistance!

```
C:\D\dmd2\windows\bin64\..\..\src\phobos\std\range\package.d(4560): Error: mutable method `app.Negative!(Take!(FibonacciSeries)).Negative.front` is not callable using a `const` object
                 return _current.front;
                        ^
c:\dev\D\81 - 
90\c82_1e_InputRange_does_support_cycle_with_save\source\app.d(44):        Consider adding `const` or `inout` here
     auto front()
          ^
C:\D\dmd2\windows\bin64\..\..\src\phobos\std\range\package.d(4695): Error: template instance `std.range.Cycle!(Negative!(Take!(FibonacciSeries)))` error instantiating
     else return Cycle!R(input);
                 ^
c:\dev\D\81 - 
90\c82_1e_InputRange_does_support_cycle_with_save\source\app.d(9):        instantiated from here: `cycle!(Negative!(Take!(FibonacciSeries)))`
             .cycle      // ← compilation ERROR
             ^
```

source/app.d
```
import std.stdio;
import std.range;

void main()
{
	writeln(FibonacciSeries()
			.take(5)
			.negative
			.cycle      // ← compilation ERROR
			.take(10));
}

struct FibonacciSeries
{
	int current = 0;
	int next = 1;
	enum empty = false;
	int front() const
	{
		return current;
	}

	void popFront()
	{
		const nextNext = current + next;
		current = next;
		next = nextNext;
	}

	FibonacciSeries save() const
	{
		return this;
	}
}

struct Negative(T) if (isInputRange!T)
{
	T range;
	bool empty()
	{
		return range.empty;
	}

	auto front()
	{
		return -range.front;
	}

	void popFront()
	{
		range.popFront();
	}

	static if (isForwardRange!T)
	{
		Negative save()
		{
			return Negative(range.save);
		}
	}
}

Negative!T negative(T)(T range)
{
	return Negative!T(range);
}
```