A keeper is a type of factory circuit that always results in the presence of an object in the output location, whether or not the object was previously present. In many cases it is easy to construct examples by connecting multiple circuits to shoot down an object with a glider, then rebuild the object again later.
The smallest keeper circuits accomplish the same thing more directly with a lucky preliminary spark from the active reaction, which removes the existing object (if any) just before the construction occurs. Below is a useful edgy block keeper with a Herschel input. The block at the right edge is removed if it is present, then later constructed/reconstructed in exactly the same location.
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RLE: here Plaintext: here