# File lib/uri_template/rfc6570/expression.rb, line 67
    def expand( vars )
      result = []
      @variable_specs.each do | var, expand , max_length |
        if Utils.def? vars[var]
          result.push(*expand_one(var, vars[var], expand, max_length))
        end
      end
      if result.any?
        return (self.class::PREFIX + result.join(self.class::SEPARATOR))
      else
        return ''
      end
    end