Deep into HBO’s thoroughly engaging documentary Street Gang: How We Got To Sesame Street, longtime Sesame Street producer and songwriter Christopher Cerf sits down at a piano to play a whimsical little Beatles pastiche. As he sings “Letter B,” a song barely shifted musically from “Let It Be,” director Marilyn Agrelo cuts between him and the 1981 Sesame Street segment where mop-haired, antenna-sporting Muppets perform the song, backed by a drum kit that gives their band name as “The Beetles.” The segment wraps, and Cerf chuckles a bit and casually adds, “That was a $5 million lawsuit.” And then the documentary moves on.