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propagating cluster sizes

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In some applications, sequences are clustered in two or two or more passes by different USEARCH commands and/or by other programs. Sometimes, the size of a cluster is required in terms of the number of sequences that were provided to the first stage of a pipeline. For example, 16S reads might dereplicated then clustered into OTUs by cluister_otus.

To handle multi-step clustering, USEARCH provides a mechanism to propagate cluster size annotations. If the ‑sizein option is specified, input sequences are required to have a size annotation. If the ‑sizeout option is specified, size annotations are added to the output labels. If both -sizein and -sizeout are given, then the output size for a cluster takes into account the input sizes.

Typical use is:

1. First clustering or dereplication step in the pipeline uses -sizeout.

2. Subsequent clustering steps use both -sizein and -sizeout.
 
If another program is used before the first USEARCH step, then it is up to you to write scripts to produce size annotations for USEARCH.