Richie Havens improvised song a Woodstock 1969.He
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The show was late in starting, and they were feeling pressure to start the music. We played for nearly three hours as they were still building the stage around me. I sang every song I knew, and when they asked me to go back on one more time, I improvised "Freedom."
http://www.cnn.com/2013/04/23/opinion/havens-his-own-words/
https://www.youtube.com/watch?v=rynxqdNMry4