m <- matrix(rpois(25, 1), 5, 5)
arr_to_coo(m, 0L)
stopifnot(identical(do.call(arr_from_coo, arr_to_coo(m, 0L)), m))
stopifnot(length(arr_to_coo(m, NULL)$x) == 25) # No baseline
m[sample.int(25L, 2L)] <- NA
m
arr_to_coo(m, 0L) # Return NAs
arr_to_coo(m, 0L, na.rm = TRUE) # Drop NAs
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