Fixed #48 : RelationalExpressionParser: Implement relational operator parser

Fixed #49 : ExpressionParser: Implement ExpressionParser
Fixed #50 : Implement ReteConditionExprVisitor
Fixed #51 : Implement PythonConditionExprVisitor
Fixed #52 : SheerkaConceptManager: I can get and set concept property

src/core/global_symbols.py

@@ -1,4 +1,5 @@
 # events
+from enum import Enum
 
 EVENT_CONCEPT_PRECEDENCE_MODIFIED = "evt_cp_m"
 EVENT_RULE_PRECEDENCE_MODIFIED = "evt_rp_m"
@@ -69,3 +70,12 @@ class ErrorObj:
 
 
 CURRENT_OBJ = "__obj"
+
+
+class SyaAssociativity(Enum):
+    Left = "left"
+    Right = "right"
+    No = "No"
+
+    def __repr__(self):
+        return self.value