code: def _merge_fields(a, b):
      
  
      a_names = set(x[0] for x in a)
      b_names = set(x[0] for x in b)
      a_keep = a_names - b_names
  
      fields = []
  
      for name, field in a:
          if name in a_keep:
              fields.append((name, field))
  
      fields.extend(b)
  
      return fields
docs: Merge two lists of fields.
  
      Fields in `b` override fields in `a`. Fields in `a` are output first.