diff --git a/pymath/calculus/str2tokens.py b/pymath/calculus/str2tokens.py
index 4456ea3..de06b9d 100644
--- a/pymath/calculus/str2tokens.py
+++ b/pymath/calculus/str2tokens.py
@@ -102,11 +102,11 @@ def in2post_fix(infix_tokens):
     @param infix_tokens: the infix list of tokens to transform into postfix form.
     @return: the corresponding postfix list of tokens.
 
-    >>> in2post_fix([op.par, 2, op.add, 5, op.sub, 1, ')', op.div, op.par, 3, op.mul, 4, ')'])
+    >>> in2post_fix(['(', 2, '+', 5, '-', 1, ')', '/', '(', 3, '*', 4, ')'])
     [2, 5, 1, '-', '+', 3, 4, '*', '/']
-    >>> in2post_fix([op.sub1, op.par, op.sub1, 2, ')'])
+    >>> in2post_fix(['-', '(', '-', 2, ')'])
     [2, '-', '-']
-    >>> in2post_fix([op.sub1, op.par, op.sub1, 2, op.add, 3, op.mul, 4, ')'])
+    >>> in2post_fix(['-', '(', '-', 2, '+', 3, '*', 4, ')'])
     [2, '-', 3, 4, '*', '+', '-']
     """
     # Stack where operator will be stocked