code: def StartVector(self, elemSize, numElems, alignment):
          
  
          self.assertNotNested()
          self.nested = True
          self.Prep(N.Uint32Flags.bytewidth, elemSize*numElems)
          self.Prep(alignment, elemSize*numElems)  # In case alignment > int.
          return self.Offset()
docs: StartVector initializes bookkeeping for writing a new vector.
  
          A vector has the following format:
            - <UOffsetT: number of elements in this vector>
            - <T: data>+, where T is the type of elements of this vector.