begin :: Pattern Double -> ControlPattern
begin turns a number pattern into a control pattern that changes the start position of a sample.
begin takes a pattern of numbers from 0 to 1. It skips the beginning of each sample, e.g. 0.25 to cut off the first quarter from each sample, 0.5 to cut off the first half, and so on:
d1 $ s "rave/2" # begin "<0 0.25 0.5>"
You can use end to alter the playback end position of a sample.