reduce_cluster() and map_cluster() apply the data reduction to the targets
found in the director step. They only do so if the metric is above the
threshold, however. reduce_cluster() is for functions that return vectors
while map_cluster() is for functions that return data.frames. If you're
using as_reducer(), there's no need to call these functions directly.
a function to reduce the data to either a vector or a data.frame
first_match
logical. Should the partition algorithm stop when it finds
a reduction that is equal to the threshold? Default is TRUE for reducers
that return a data.frame and FALSE for reducers that return a vector
rewind
logical. Should the last target be used instead of the
current target?