cancelTask doesn't always work

i33 : t = schedule createTask (() -> sleep 2) ; cancelTask t

i35 : t

o35 = <<task, running, cancellation requested>>

o35 : Thread

i36 : t

o36 = <<task, running, cancellation requested>>

o36 : Thread

i37 : t

o37 = <<task, running, cancellation requested>>

o37 : Thread

i38 : t

o38 = <<task, running, cancellation requested>>

o38 : Thread

i39 : t

o39 = <<task, running, cancellation requested>>

o39 : Thread
