code: def set_virtualchain_field(cls, opdata, virtualchain_field, value):
          
          assert virtualchain_field in RESERVED_KEYS, 'Invalid field name {} (choose from {})'.format(virtualchain_field, ','.join(RESERVED_KEYS))
          opdata[virtualchain_field] = value
docs: Set a virtualchain field value.
          Used by implementations that generate extra consensus data at the end of a block