fatdiag.R
In diagonals: Block Diagonal Extraction or Replacement

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library(diagonals)
fatdiag(12, steps=3)

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( m <- matrix(111, nrow=6, ncol=9) )
fatdiag(m, steps=3) <- 5
m

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fatdiag(m, steps=3)

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fatdiag(12, size=4)

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fatdiag(12, size=c(3,4) )