it <- iteror(c("A", "B", "C"))
it2 <- iteror("D")
it3 <- iteror(c("E", "F"))
as.list(i_roundrobin(it, it2, it3)) # A D E B F C
it_rr <- i_roundrobin(1:3, 4:5, 7:10)
as.list(it_rr) # 1 4 7 2 5 8 3 9 10
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