def get_typeof_error name
      pspec = get_pspec name
      raise Vips::Error unless pspec

      pspec[:value][:value_type]
    end
def get_typeof_error name
      pspec = get_pspec name
      raise Vips::Error unless pspec

      pspec[:value][:value_type]
    end
return a gtype, raise an error on not found