Add doctests to primelib.py (#10242)

* Update primelib.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

maths/primelib.py

@@ -574,6 +574,11 @@ def fib(n):
  """
  input: positive integer 'n'
  returns the n-th fibonacci term , indexing by 0
+
+ >>> fib(5)
+ 8
+ >>> fib(99)
+ 354224848179261915075
  """
 
  # precondition
@@ -589,3 +594,9 @@ def fib(n):
  fib1 = tmp
 
  return ans
+
+
+if __name__ == "__main__":
+ import doctest
+
+ doctest.testmod()