reserve a,b,n for Element of NAT;

theorem
  for a,b,n being Element of NAT holds GenFib(a,b,n + 3) - GenFib(a,b,n)
  = 2*GenFib(a,b,n +1)
proof
  let a,b,n be Element of NAT;
  GenFib(a,b,n + 3) - GenFib(a,b,n) = GenFib(a,b,n+1)+ GenFib(a,b,n+2)-
  GenFib(a,b,n) by Th35
    .=GenFib(a,b,n+1)+ (GenFib(a,b,n)+GenFib(a,b,n+1))- GenFib(a,b,n) by Th34
    .= 2*GenFib(a,b,n +1);
  hence thesis;
end;
