A matrix multiplier multiplies the signal output from a first adder by an inverse matrix T\u22121 of a partial matrix T of a parent parity check matrix, and outputs the multiplication result to a first switch. A curtain is positioned in a housing adjacent to an upper limit of an opening such as a hoist access-way. Each disk surface is coupled to an array of magnets arranged as matching magnetic pairs on two adjacent disk surfaces so as to create a plurality of magnetic fields between the matching magnetic pairs. The plug in one aspect is selectively set and released by a settingreleasing tool. The first lead has a first transition portion and a first bottom portion with a first cutting member, and the second lead having a second transition portion and a second bottom portion with a second cutting member. The trim blade has a trim blade edge having a first section and a second section, the second section being angled with respect to the first section.