# File lib/uri_template/rfc6570/expression.rb, line 253
    def pair(key, value, max_length = 0, &block)
      ek = key
      if block
        ev = value.map(&block).join(self.class::LIST_CONNECTOR) 
      else
        ev = escape(value)
      end
      if !self.class::PAIR_IF_EMPTY and ev.size == 0
        return ek
      else
        return ek + self.class::PAIR_CONNECTOR + cut( ev, max_length )
      end
    end