Premise: "A Hard Day's Night" is a song by the English rock band the Beatles. Credited to Lennon–McCartney, it was written by John Lennon, with some collaboration from Paul McCartney. It was released on the film soundtrack of the same name in 1964. It was also released in the UK as a single, with "Things We Said Today" as its B-side.
Hypothesis: John Lennon is the one who writes for Lennon–McCartney.