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Sow[expr] appends expr to a list of expressions that will be returned by the nearest enclosing Reap.
Sow[expr, tag] appends expr to a list of expressions, associated with the particular value of tag, that will be returned by the nearest enclosing Reap[body, patt] for which tag matches patt.
Sow[expr, , , ...] sows expr multiple times, to lists associated with the .
To use a list as a tag, enclose it within a list. For example, Sow[expr, a] sows expr with an association to the tag a.
See notes for Reap.
Note: this is an experimental feature, and in future versions of Mathematica it may not be supported, or may have a different specification.