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Originally posted by ark13The not seeing the quicker mate is due to the engine finding a mate in say n+1 and not analysing further to find the mate in n. You can force it to find the shortest mate by putting it in mate search mode (caution this is very slow as it analyses every single move with no pruning (but you can be sure if it says there is no mate in 10 there isn't one (although it won't tell you that for two hours))).
Maybe it's just something weird with Shredder. I can't remember if I've seen my other engines doing it. Fritz found it quickly, without the mate in 150. However, my Fritz is behaving strangely too in that on white's move, he shows Rc7 ...[text shortened]... Shredder only sees mate in 7 before Rc7. It was mate in 6 after.
EDIT: However hash tables should enable it to find the shorter mate after being shown it from a later position. At least in the newest version of Shredder.