## ----echo=FALSE---------------------------------------------------------------
set.seed(1337)
m <- matrix(sample(0:1, 20, replace = TRUE), nrow = 4)
rownames(m) <- paste0("doc", 1:4)
colnames(m) <- c("partir", "un", "jour", "sans", "retour")
m

## ----echo=FALSE---------------------------------------------------------------
tmp <- rbind(colSums(m[1:2,]), colSums(m[3:4,]))
rownames(tmp) <- c("doc1 + doc2", "doc3 + doc4")
tmp