code: def PrependStructSlot(self, v, x, d):
          
  
          N.enforce_number(d, N.UOffsetTFlags)
          if x != d:
              self.assertStructIsInline(x)
              self.Slot(v)
docs: PrependStructSlot prepends a struct onto the object at vtable slot `o`.
          Structs are stored inline, so nothing additional is being added.
          In generated code, `d` is always 0.