How literal is this phrase?

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donkey pump:

Definitions:

  1. a supplementary pump available if needed

Examples:

  1. the original 85 were withdrawn from service . Around this time , injectors replaced the donkey pumps , with the feedwater heaters removed , although no . 314 retained its donkey pump
  2. donkey pumps , with the feedwater heaters removed , although no . 314 retained its donkey pump until rebuilt by the Southern Railway in 1931 . The introduction of Adams class 415
  3. want to see and understand middle-France . We arrive in an area clattered with nodding donkeys pumping oil out . I wonder whether our 4 -stroke engines would work on crude oil

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