seq (flu, index = TRUE)
seq_along (flu)
seq (flu, length.out = 3, index = TRUE) # return value is numeric, not integer!
seq (flu, by = 2, index = TRUE) # return value is numeric, not integer!
plot (flu, col = "darkgray")
plot (seq (flu, by = 2), add = TRUE, col= "red")
plot (seq (flu, length.out = 2), add = TRUE, col= "blue")
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