x1 <- 1:5
x2 <- 6:10
x3 <- 50:60
sampleAcross(x1, x2, x3)
sampleAcross(x1, x2, x3, replace=TRUE)
a <- data.frame(x=1:10, y=letters[1:10])
b <- data.frame(x=11:20, y=letters[11:20])
sampleAcross(a, b, by='y')
sampleAcross(a, b)
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