Question: Phase potrait plot

I am currently studying the phase portrait a differential equation: diff(y(t), t, t) = -piecewise((diff(y(t), t))*y(t) <><> 0 <><><> Initial conditions are: y(0) = 1; , diff(y(0), t) = 0; The phase portrait plots diff(y(t),t) with respect to y(t). I achieved the phase portrait using Simulink; with a few gain blocks, a switch and a X-Y scope, easily enough. When i was done, i wondered if i could analyze the equation with a more mathematical approach. I decided to try and plot the phase portrait using Maple 11. I have tried various commands to solve the differential equation with Maple 11 but so far i have confused myself. Any suggestions are welcomed!!
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