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Tip #1: Put a few ice cubes with a pack of frozen veggie over it – after the ice melts, fluff the area with an old toothbrush.
Tip #2: Use a combination of steam (a warm iron over a damp tea towel whose colour won’t run) and a stiff hairbrush. You are supposed to alternately steam it, then rough up the pile with the brush till its gone. We just waited a few months, and the carpet recovered.