[racket] generating 0 or more than 1 additions in the body of for/list, for/vector, for/hash

When using  the various versions of for like for/list; I often encounter
situations where the body must not add any value to put into the generated
list  (useful when one needs to filter and map simultaneously) or must
generate more than value. I guess the way to do this would be to treat the
result of the body as a list of additions and then concatenate all these
lists. Something similar would be needed for for for/hash(taking care of the
multiple values issue),for/and, for/vector etc.
  I just wanted to know, because this seems like a common enough situation,
if there are some standard commands or libraries which implement the
modified versions of these commands.
thanks,
Harsha
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