itself. "Progressive jazz" was a broad category of music that included bebop-influenced "art music" arrangements used by big bands such as those led by Boyd Raeburn, Charlie Ventura, Claude Thornhill, and Stan Kenton, and the cerebral harmonic explorations of smaller groups such as those led by pianists Lennie Tristano and Dave Brubeck. Voicing experiments based on bebop harmonic devices were used by Miles Davis and Gil Evans for the groundbreaking "Birth of the Cool" sessions in 1949 and 1950. Musicians who followed the stylistic doors opened by Davis, Evans, Tristano, and Brubeck formed the core of the cool jazz and "west coast jazz" movements of the early 1950s. By the mid-1950s musicians began to be influenced by music theory proposed by George Russell. Those who incorporated Russell's ideas into the bebop foundation defined the post-bop movement that later incorporated modal jazz into its musical language. Hard bop was a simplified derivative of bebop introduced by Horace Silver and Art Blakey in the mid-1950s. It became a major influence until the late 1960s when free jazz and fusion jazz gained ascendancy. The neo-bop movement of the 1980s and 1990s revived the influence of bebop, post-bop, and hard bop styles after the free jazz and fusion eras. Bebop style also influenced the Beat Generation whose spoken-word style drew on African-American "jive" dialog, jazz rhythms, and whose poets often employed jazz musicians to accompany them. Jack Kerouac would describe his writing in On the Road as a literary translation of the improvisations of Charlie Parker and Lester Young. The "beatnik" stereotype borrowed heavily from the dress and mannerisms of bebop musicians and followers, in particular the beret and lip beard of Dizzy Gillespie and the patter and bongo drumming of guitarist Slim Gaillard. The bebop subculture, defined as a non-conformist group expressing its values through musical communion, would echo in the attitude of the psychedelia-era hippies of the 1960s. Fans of bebop were not restricted to the United States; the music also gained cult status in France and Japan. More recently, hip-hop artists (A Tribe Called Quest, Guru) have cited bebop as an influence on their rapping and rhythmic style. As early as 1983, Shawn Brown rapped the phrase "Rebop, bebop, Scooby-Doo" toward the end of the hit "Rappin' Duke". Bassist Ron Carter collaborated with A Tribe Called Quest on 1991's The Low End Theory, and vibraphonist Roy Ayers and trumpeter Donald Byrd