code: def walk(self, node):
          "Walk a parse tree, calling visit for each node."
          node = self.visit(node)
  
          if node is None:
              return None
  
          if isinstance(node, parso.tree.BaseNode):
              walked = map(self.walk, node.children)
              node.children = [child for child in walked if child is not None]
  
          return nodf walk(self, node):
          "Walk a parse tree, calling visit for each node."
          node = self.visit(node)
  
          if node is None:
              return None
  
          if isinstance(node, parso.tree.BaseNode):
              walked = map(self.walk, node.children)
              node.children = [child for child in walked if child is not None]
  
          return node
docs: Walk a parse tree, calling visit for each node.