# File lib/runt/temporalexpression.rb, line 108
  def to_s
    if !@expressions.empty? && block_given?
      first_expr, next_exprs = yield
      result = '' 
      @expressions.map do |expr|
        if @expressions.first===expr
          result = first_expr + expr.to_s
        else
         result = result + next_exprs + expr.to_s
        end 
      end
      result
    else
      'empty'
    end
  end