Two DJs began the lesson, in contrasting style. Afrika Bambaataa was once the samurai-clad leader of the Soul Sonic Force, the man who with "Planet Rock" allied hip-hop to Kraftwerk, anticipating techno. On Monday, he took the stage alone, almost unnoticed, absentmindedly mixing records with a modernistic sheen, as if his legend was a long way behind him, as if the present was all that mattered.
Grandmaster Flash took his mystique more seriously. The music he made used fragments of hip-hop's first sources - James Brown, Chic, old soul. His scratching was as warm, as redolent of history as the records he sampled, but his unsentimental presence kept the mix alive until he cursed the sound and was gone, the impression of erratic genius careering by confirmed.
The Jungle Brothers must have wished to obliterate parts of their past - the dark days when their leading role in the Native Tongues, the movement they formed with De la Soul and A Tribe Called Quest, had left them stranded as rap toughened and moved on. They peered back anyway, through their long, influential history of conscious funk.
But it was their new single, "Brain", which brought them into focus. It was almost a love song, wistful for a future which, having survived rap's attrition this long, they could almost believe in.
Then they led the crowd in an old chant: "Peace, unity, love and havin' fun," over a sea of raised peace-signs. "This is how we used to do it," they said defiantly. "This is how we still do it." And this is why the Native Tongues recently reformed, the same exhaustion at black bloodshed that De La Soul's last record caught. Waving peace-signs in a tent for once seemed a valid response.
In the subtext of the day, Ice-T might have seemed part of the problem, and he knew how it looked. "I took a perfectly innocent music, and added drugs, bitches, money, gunshots," he confessed. "Totally fucked up hip- hop, didn't I?" More than anyone else, Ice-T took all the conflicts in rap's history on stage with him. He played up his hustler image. He hit brutal heights in descriptions of death worthy of James Ellroy ("You end up shot, wind up lookin' at a zip-lock"), then warned against drugs. Freestyling what he'd do to "white bitches", he paused to explain why women shouldn't be offended.
His act walked a razor's edge, leaning one way then the other, coaxing everyone with him. Posing as hip-hop's bad seed, he smoothed over the shuddering leaps between each of the previous acts' eras, emphasising continuities. He ended with a new rap as stuffed with positivity as The Jungle Brothers. As a last touch, he bowed and doffed his hat.
Treading round the fault-lines on which so many think hip-hop is ready to crack, he completed the day's job - to show that hip-hop can not only zoom in on ghetto catastrophe, but rise above it: a musical nation which doesn't deserve to have its heroes in the ground, and was relieved to find so many still moving.