code: def resolve_att(a, fallback):
      
      if a is None:
          return fallback
      if a.background in ['default', '']:
          bg = fallback.background
      else:
          bg = a.background
      if a.foreground in ['default', '']:
          fg = fallback.foreground
      else:
          fg = a.foreground
      return AttrSpec(fg, bg)
docs: replace '' and 'default' by fallback values