A filter stream is a stream of spaceships in which there are periodic gaps in the stream. This can thin out another crossing stream by deleting the spaceships in the second stream except where the gaps occur. The filter stream is not affected by the deletions, so the same stream can thin out multiple other streams. The Caterpillar uses filter streams of MWSSes in which there is a gap every 6 spaceships. Below is part of a filter stream that thins a glider stream by 2/3.
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RLE: here Plaintext: here