title: Program to generate fibonacci series in GNU Prolog is giving an instantiation error
body: <p>This is my code:-</p>
  
  <pre><code>fib(0,0).
  fib(1,1).
  fib(F,N) :-
      N&gt;1,
      N1 is N-1,
      N2 is N-2,
      F is F1+F2,
      fib(F1,N1),
      fib(F2,N2),
      write(F," ,").
  </code></pre>
  
  <p>On consulting in GNU Prolog, I am getting:</p>
  
  <pre><code>| ?- consult('C:/GNU-Prolog/bin/fib.pl').
  compiling C:/GNU-Prolog/bin/fib.pl for byte code...
  C:/GNU-Prolog/bin/fib.pl compiled, 3 lines read - 1253 bytes written, 15 ms
  
  yes
  | ?- fib(F,2).
  uncaught exception: error(instantiation_error,(is)/2)
  </code></pre>