Do some refactor with multiple left associative operators

smnp/ast/node/operator.py

@@ -0,0 +1,15 @@
+from smnp.ast.node.model import Node
+
+
+class OperatorNode(Node):
+    def __init__(self, pos):
+        super().__init__(pos)
+        self.children = [None]
+
+    @property
+    def value(self):
+        return self[0]
+
+    @value.setter
+    def value(self, value):
+        self[0] = value