code: def visit_Subscript(self, node):
      
      if not self.target:
        return node
  
      template_ = tangents.tangents[node.__class__]
      tangent_node = template.replace(
          template=template_,
          replace_grad=template.Replace.TANGENT,
          namer=self.namer,
          x=node,
          z=self.target)
      return tangent_node
docs: Tangent of e.g.
      x = y[i]