Lazy range, extract only Nth element, set range size constraint?

[biocyberman via Digitalmars-d-learn]

Following is the code for a more generalized Fibonacci range.

Questions:

1. How do I get only the value of the Nth (i.e. N = 25) element 
in an idiomatic way?

2. Can I set constraints in the range so that user gets warning 
if he asks for Nth element greater than a limit, say N> 30; or if 
the actually range value at N is greater than datatype limit 
(e.g. max long)? Maybe this should be done outside of the range, 
i.e. do check before accessing the range?

     #!/usr/bin/env rdmd
     import std.stdio : writeln;
     long multifactor = 4;
     int elemth = 25;

     struct FibonacciRange
     {
       long first = 1;
       long second = 1;

       bool empty() const @property
       {
        // how to stop at n = 30?
         return false;
       }

       void popFront()
       {
         long tmp = 0;
         tmp = first*multifactor + second;
         first = second;
         second = tmp;
       }

       long front() const @property
       {
         return first;
       }
     }

     void main()
     {
         import std.range : take;
         import std.array : array;

         FibonacciRange fib;

         auto fib10 = take(fib, elemth);
         long[] the10Fibs = array(fib10);
     }