Question: What unreleased song's tune was re-used in a song which appeared on the 1971 album 'Imagine' by John Lennon?

Context paragraphs:
Oh My Love: "Oh My Love" is a song written by John Lennon and Yoko Ono that appeared on Lennon's "Imagine" album in 1971.  The song is the only songwriting collaboration on "Imagine."

Dreamer (Ozzy Osbourne song): "Dreamer" is the third track from Ozzy Osbourne's album "Down to Earth", which was released on October 16, 2001.  The single peaked at number 10 on "Billboard'"s Mainstream Rock Tracks.  The song describes the rockstar's vision of a better world for his children, where they are happy and safe.  In the liner notes to Osbourne's "Prince of Darkness" box set, Ozzy refers to this song as his version of John Lennon's "Imagine".  A second video was also produced, featuring highlights from "The Osbournes".  The Japanese and European press of the single also include a slightly different version of the song Dreamer, dubbed on the single "Dreamer (acoustic)", and a previously unreleased song titled "Black Skies".

Imagine (1972 film): Imagine is a 1972 television film by John Lennon and Yoko Ono, filmed mostly at their Tittenhurst Park home in Ascot, England, during 1971, and intended for television.  All the songs from Lennon's "Imagine" album appear in the soundtrack, and also the songs "Mrs. Lennon" and "Don't Count the Waves", from Ono's album "Fly".

It's So Hard: "It's So Hard" is a song written and performed by John Lennon which first appeared on his 1971 album "Imagine".  Shortly after the album's release, the song was released as the B-side to the single "Imagine."  In Mexico it was released on an EP with "Imagine," "Oh My Love" and "Gimme Some Truth."  In 1986, a live performance from 30 August 1972 was released on Lennon's live album "Live in New York City".

How Do You Sleep? (John Lennon song): "How Do You Sleep?"  is a song by English musician John Lennon from his 1971 album "Imagine".  The song makes angry and scathing remarks aimed at his former Beatles bandmate and songwriting partner, Paul McCartney.  Lennon wrote the song in response to what he perceived as personal slights by McCartney on the latter's "Ram" album.  The track includes a slide guitar solo played by George Harrison, also a former member of the Beatles.

Mother Nature's Son: "Mother Nature's Son" is a song written primarily by Paul McCartney (credited to Lennon–McCartney) and released by the Beatles on "The Beatles" ("the White Album").  It was inspired by a lecture given by the Maharishi Mahesh Yogi while the Beatles were in India.  The same lecture inspired Lennon's unreleased song "Child of Nature", the tune of which he later re-used for "Jealous Guy".

Crippled Inside: "Crippled Inside" is a song written and performed by John Lennon from his 1971 album "Imagine".

Jealous Guy: "Jealous Guy" is a song written and performed by John Lennon, which first appeared on his 1971 album "Imagine".  It is one of the most commonly recorded Lennon songs, with at least 92 other recorded versions, the most notable being Roxy Music's version, which reached number one in several countries three months after John Lennon's death.

The Magic Key (song): "The Magic Key" is a 2002 hip-hop/electronic song recorded by French animated feature One-T + Cool-T.  It was the third single from the album "The One-T ODC" and was released in April 2003.  The song samples 'Má Hra' (1971 album 'Nová Syntéza') by Blue Effect, a Czechoslovakian progressive rock band.

Gimme Some Truth: "Gimme Some Truth" (originally spelled "Give Me Some Truth") is a protest song written and performed by John Lennon.  It was first released on his 1971 album "Imagine".  "Gimme Some Truth" contains various political references emerging from the time it was written, during the latter years of the Vietnam War.

Supporting paragraphs: Mother Nature's Son Jealous Guy
Answer: Child of Nature