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# first code block
knitr::opts_knit$set(root.dir = "..")
readLines("test.txt")

## Warning in file(con, "r"): cannot open file 'test.txt': No such file or
## directory

## Error in file(con, "r"): cannot open the connection

# next code block
readLines("test.txt")

## [1] "hello" "world"