def require(*keys)
      permitted_keys.merge keys
      required_keys = keys.to_set
      unless (missing_keys = required_keys - hash_keys).empty?
        error "required #{terms} #{missing_keys.to_a} missing" + postscript
      end
      self
    end
def require(*keys)
      permitted_keys.merge keys
      required_keys = keys.to_set
      unless (missing_keys = required_keys - hash_keys).empty?
        error "required #{terms} #{missing_keys.to_a} missing" + postscript
      end
      self
    end
Specifies keys that must exist