It's just an ellipsis. Or, as my first Google hit said: "the omission from speech or writing of a word or words that are superfluous or able to be understood f

It's just an ellipsis. Or, as my first Google hit said: "the omission from speech or writing of a word or words that are superfluous or able to be understood from contextual clues." So: I would rather [have] a bus any day.