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x[[(5,3)]]

An R tip: Did you know that x[[c(5,3)]] is the same as x[[5]][[3]]?

I should make more thorough use of this.

In the help file for [[:

[[ can be applied recursively to lists, so that if the single index i is a vector of length p, alist[[i]] is equivalent to alist[[i1]]...[[ip]] providing all but the final indexing results in a list.

I never knew this; I came across it when playing around (i.e., not paying proper attention) in the back of the room at an R course.

Did you know that [[ had a help file? Type ?"[["