.adds x to acc and returns acc

A high-performance combining fn that yields the catenation of the
reduced values. The result is reducible, foldable, seqable and
counted, providing the identity collections are reducible, seqable
and counted. The single argument version will build a combining fn
with the supplied identity constructor. Tests for identity
with (zero? (count x)). See also foldcat.

Elides the first n values from the reduction of coll.

Retains values in the reduction of coll for which (pred val)
  returns logical true. Foldable.

Takes any nested combination of sequential things (lists, vectors,
  etc.) and returns their contents as a single, flat foldable
  collection.

Reduces a collection using a (potentially parallel) reduce-combine
strategy. The collection is partitioned into groups of approximately
n (default 512), each of which is reduced with reducef (with a seed
value obtained by calling (combinef) with no arguments). The results
of these reductions are then reduced with combinef (default
reducef). combinef must be associative, and, when called with no
arguments, (combinef) must produce its identity element. These
operations may be performed in parallel, but the results will
preserve order.

Note: Performing operations in parallel is currently not implemented.

Equivalent to (fold cat append! coll)

Given a foldable collection, and a transformation function xf,
returns a foldable collection, where any supplied reducing
fn will be transformed by xf. xf is a function of reducing fn to
reducing fn.

Applies f to every value in the reduction of coll. Foldable.

Applies f to every value in the reduction of coll, concatenating the result
  colls of (f val). Foldable.

Builds a combining fn out of the supplied operator and identity
constructor. op must be associative and ctor called with no args
must return an identity value for it.

Like core/reduce except:
  When init is not provided, (f) is used.
  Maps are reduced with reduce-kv

Given a reducible collection, and a transformation function xf,
returns a reducible collection, where any supplied reducing
fn will be transformed by xf. xf is a function of reducing fn to
reducing fn.

Removes values in the reduction of coll for which (pred val)
  returns logical true. Foldable.

Ends the reduction of coll after consuming n values.

Ends the reduction of coll when (pred val) returns logical false.