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Jim VBI have successfully used a tile nippers to clip a corner off. (These are usually used to cut little pieces of ceramic tile.) Little "bites" to start. Once you have an opening, the flat head to pry it open is the best.
They look like a pliers with a cutting blade turned sideways. You can pick them up at any Lowes, Home Depot, or hardware store.
http://www.amazon.com/Diamond-Tough-GT-Tile-Nippers/dp/B000MDA3MA