Question: How to change the default increment in functions like add()

Hi, I got a simple question. I want to use the add function but with an increment rather than 1. for example add(a[i], i = 1 .. 4) gives a[1]+a[2]+a[3]+a[4], whereas I want to have a[1]+a[3] for instance. Please let me know how I can define it that way. Thank you,
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