Understanding this assignment? Languages

Understanding this assignment?

In order to refresh my 20 year old experience with Haskell I am walking through https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours/Adding_Variables_and_Assignment and at one point the following line is introduced to apply op to all the params. This is to implement e.g. (+ 1 2 3 4)
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