code: def names(expr: AST) -> Set[str]:
      
      nodes = [node for node in ast.walk(expr) if isinstance(node, ast.Name)]
      loaded = {node.id for node in nodes if isinstance(node.ctx, ast.Load)}
      stored = {node.id for node in nodes if isinstance(node.ctx, ast.Store)}
      return loaded - stored
docs: Names of globals in `expr`.