def init_with_hash(hash)
      @tuple = Hash.new
      hash.each do |k, v|
        raise InvalidHashTupleKey unless String === k
        @tuple[k] = v
      end
    end
def init_with_hash(hash)
      @tuple = Hash.new
      hash.each do |k, v|
        raise InvalidHashTupleKey unless String === k
        @tuple[k] = v
      end
    end
Ensures +hash+ is a valid Tuple.