favorite favourite

What's your favourite standard library function?

Mine is Matlab's squeeze. Every time I use it I can imagine my hyperdimensional manifestation pushing the matrix until all those useless dimensions become shiny drops of acid that are slowly absorbed into the surrounding space.

My favourite function is haskell's Control.Arrow.loop. Every time I use it I can just imagine some fucking java expert programmer looking at my code and squirting in incomprehension at the higher-order magnificence.