Check that value is a list of the required type.

def Validate(self, value, **_):
    """Check that value is a list of the required type."""
    # Assigning from same kind can allow us to skip verification since all
    # elements in a RepeatedFieldHelper already are coerced to the delegate
    # type. In that case we just make a copy. This only works when the value
    # wraps the same type as us.
    if (value.__class__ is RepeatedFieldHelper and
        value.type_descriptor is self.delegate):
      result = value.Copy()

    # Make sure the base class finds the value valid.
    else:
      # The value may be a generator here, so we just iterate over it.
      result = RepeatedFieldHelper(type_descriptor=self.delegate)
      result.Extend(value)

    return result