英语翻译原文：The decision was made to err on the safe side because a complete development system specific to the chosen part was necessary,and the project budget did not permit a second such system to be purchased should the first prove inadequate.前一句还好理解,后一句里面的那个should的主语是谁?

英语翻译
原文：The decision was made to err on the safe side because a complete development system specific to the chosen part was necessary,and the project budget did not permit a second such system to be purchased should the first prove inadequate.
前一句还好理解,后一句里面的那个should的主语是谁?