Implement semantic error handling for duplicate screen declarations

compiler.py

@@ -1673,12 +1673,20 @@ def syntactical_analyzer(tokens: Sequence[Token]) -> File:
 # -- Semantics --
 
 
-# class SemanticsError(CompilerError):
-class SemanticsError(Exception):
+class SemanticError(CompilerError):
 
     _compiler_error_type = "Semantics"
 
 
+class DuplicateScreen(SemanticError):
+
+    def __init__(self, file_info: FileInfo):
+        super().__init__(
+            "Duplicate 'screen' declaration.",
+            file_info,
+        )
+
+
 class ContextOperator(Enum):
     NoOp = "0"
     Identifier = "i"
@@ -1984,13 +1992,17 @@ def semantics_analyzer(file: File) -> Context:
     graphs: list[ContextGraph] = []
     for child in file.children:
         if isinstance(child, Screen):
-            pass
+            if screen is not None:
+                raise DuplicateScreen(child.file_info)
+            screen = child
         elif isinstance(child, Constant):
             pass
         elif isinstance(child, Animation):
             pass
         elif isinstance(child, Graph):
             pass
+    if screen is None:
+        screen = Screen(file.file_info)
 
 
 if __name__ == '__main__':