'[[' and recursiveness

• '[[' operates recursively when given mulitple indexes.
• Compare:

# atomic vector with '['
x <- 1:10
x[c(1, 2)]

## [1] 1 2

y <- list(a = 1:10,
          b = 11:20)

# list with '[['
y[[c(1,2)]]   # equivalent to y[[1]][[2]]

## [1] 2