Allow SENTENCE to be marked as sentinel

Useful to insert automatic "default" code.

Author: boriel

src/symbols/sentence.py

@@ -15,13 +15,17 @@
 class SymbolSENTENCE(Symbol):
     """ Defines a BASIC SENTENCE object. e.g. 'BORDER'.
     """
-    def __init__(self, lineno: int, filename: str, keyword: str, *args):
-        """ keyword = 'BORDER', or 'PRINT'
+    def __init__(self, lineno: int, filename: str, keyword: str, *args, is_sentinel=False):
+        """  Params:
+            - keyword: BASIC sentence token like 'BORDER', 'PRINT', ...
+            - sentinel: whether this sentence was automagically added by the compiler
+                (i.e. a RETURN "" in a string function when the user does not return anything)
         """
         super(SymbolSENTENCE, self).__init__(*(x for x in args if x is not None))
         self.keyword = keyword
         self.lineno = lineno
         self.filename = filename
+        self.is_sentinel = is_sentinel
 
     @property
     def args(self):