code: def read_enum(fo, writer_schema, reader_schema=None):
      
      index = read_long(fo)
      symbol = writer_schema['symbols'][index]
      if reader_schema and symbol not in reader_schema['symbols']:
          default = reader_schema.get("default")
          if default:
              return default
          else:
              symlist = reader_schema['symbols']
              msg = '%s not found in reader symbol list %s' % (symbol, symlist)
              raise SchemaResolutionError(msg)
      return symbol
docs: An enum is encoded by a int, representing the zero-based position of the
      symbol in the schema.