code: def load_mode_builder(obs_mode, node):
      
  
      # Check 'builder' and 'builder_options'
      nval1 = node.get('builder')
  
      if nval1 is not None:
          if isinstance(nval1, str):
              # override method
              newmethod = import_object(nval1)
              obs_mode.build_ob = newmethod.__get__(obs_mode)
          else:
              raise TypeError('builder must be None or a string')
      else:
          nval2 = node.get('builder_options')
  
          if nval2 is not None:
              if isinstance(nval2, list):
                  for opt_dict in nval2:
  
                      if 'result_of' in opt_dict:
                          fields = opt_dict['result_of']
                          obs_mode.build_ob_options = ResultOf(**fields)
                          break
              else:
                  raise TypeError('builder_options must be None or a list')
  
      return obs_mode
docs: Load observing mode OB builder