def verify(partial)
      unless partial.kind_of?(Partial)
        raise ArgumentError, 'you can only push Osheet::Partial objs to the partial set'
      end
      pkey = partial_key(partial)
      self[pkey] ||= nil
      pkey
    end
def verify(partial)
      unless partial.kind_of?(Partial)
        raise ArgumentError, 'you can only push Osheet::Partial objs to the partial set'
      end
      pkey = partial_key(partial)
      self[pkey] ||= nil
      pkey
    end
verify the partial, init and return the key
  otherwise ArgumentError it up