Refactor str2in_tokens use op.available_op from Operator_set

pymath/calculus/operator/operator_set.py

@@ -54,6 +54,23 @@ class Operator_set(object):
         except KeyError:
             raise KeyError("{theOp} (arity: {arity}) is not available".format(theOp = op, arity = arity))
 
+    def available_op(self):
+        """ Get the set of available operators strings
+
+        >>> op = Operator_set()
+        >>> op.available_op()
+        set()
+        >>> op.store_operator(Add())
+        >>> '+' in op.available_op()
+        True
+        >>> '*' in op.available_op()
+        False
+        >>> op.store_operator(Mul())
+        >>> '*' in op.available_op()
+        True
+        """
+        return set([i[0] for i in self._operators])
+
     def can_be_operator(cls, symbole):
         r""" 
         Tell if the symbole can be an operator