code: func (d *Dataset) Map(name string, mapperId gio.MapperId) *Dataset {
  	ret, step := add1ShardTo1Step(d)
  	step.Name = name + ".Map"
  	step.IsPipe = false
  	step.IsGoCode = true
  
  	ex, _ := os.Executable()
  
  	mapper, _ := gio.GetMapper(mapperId)
  	step.Description = mapper.Name
  
  	var args []string
  	args = append(args, os.Args[1:]...)
  	args = append(args, "-gleam.mapper", string(mapperId))
  	step.Command = &script.Command{
  		Path: ex,
  		Args: args,
  	}
  	return ret
  }
docs: // Mapper runs the mapper registered to the mapperId.
  // This is used to execute pure Go code.