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       Takes two regexes and a snippet of code. Creates an aggregator that
       creates a map. Keys in the map correspond to pairs of fields chosen by
       matching the regexes against the fields from input records. Values in the
       map are produced by aggregators which the snippet must act as a factory
       for ($f1 is the first field, $f2 is the second field).
 
       Example(s):
          To find the covariance of all x-named fields with all y-named fields:
             for_field(qr/^x/, qr/^y/, <<covar($f1, $f2)>>)
 
    map_reduce_agg(<snippet>, <snippet>[, <snippet>])
    map_reduce_aggregator(<snippet>, <snippet>[, <snippet>])
    mr_agg(<snippet>, <snippet>[, <snippet>])
    mr_aggregator(<snippet>, <snippet>[, <snippet>])
       Take a map snippet, a reduce snippet, and an optional squish snippet to
       produce an ad-hoc aggregator based on map reduce. The map snippet takes $r
       representing a record and returns its mapped value. The reduce snippet
       takes $a and $b representing two mapped values and combines them. Finally,
       the squish snippet takes a mapped value $a representing all the records
       and produces the final answer for the aggregator.
 
       Example(s):
          Track count and sum to produce average:
             mr_agg(<<[1, {{ct}}]>>, <<[$a->[0] + $b->[0], $a->[1] + $b->[1]]>>, <<$a->[1] / $a->[0]>>)
 
    rec()
    record()
       A valuation that just returns the entire record.
 
    snip(snip)
       Takes a snippet and returns both the snippet and the snippet as a
       valuation. Used to distinguished snippets from scalars in cases where it
       matters, e.g. min('{{x}}') interprets it is a keyspec when it was meant to
       be a snippet (and then a valuation), min(snip('{{x}}')) does what is
       intended. This is used internally by <<...>> and in fact <<...>> just
       translates to snip('...').
 
    subset_agg(<snippet>, <aggregator>)
    subset_aggregator(<snippet>, <aggregator>)
       Takes a snippate to act as a record predicate and an aggregator and
       produces an aggregator that acts as the provided aggregator as run on the
       filtered view.
 
       Example(s):
           An aggregator that counts the number of records with a time not above 6 seconds:
              subset_agg(<<{{time_ms}} <= 6000>>, ct())
 
    type_agg(obj)
    type_scalar(obj)
    type_val(obj)
       Force the object into a specific type. Can be used to force certain
       upconversions (or avoid them).
 
    valuation(sub { ... })
    val(sub { ... })
       Takes a subref, creates a valuation that represents it. The subref will
       get the record as its first and only argument.
 
       Example(s):
          To get the square of the "x" field:
             val(sub{ $[0]->{x} ** 2 })
 
    xform(<aggregator>, <snippet>)
       Takes an aggregator and a snippet and produces an aggregator the
       represents invoking the snippet on the aggregator's result.
 
       Example(s):
          To take the difference between the first and second time fields of the record collection:
             xform(recs(), <<{{1/time}} - {{0/time}}>>)
 
 Help from: --help-keygroups:
 KEY GROUPS
    SYNTAX: !regex!opt1!opt2... Key groups are a way of specifying multiple
    fields to a recs command with a single argument or function. They are
    generally regexes, and have several options to control what fields they
    match. By default you give a regex, and it will be matched against all first
    level keys of a record to come up with the record list. For instance, in a
    record like this:
 
    { 'zip': 1, 'zap': 2, 'foo': { 'bar': 3 } }
 
    Key group: !z! would get the keys 'zip' and 'zap'
 
    You can have a literal '!' in your regex, just escape it with a \.
 
    Normally, key groups will only match keys whose values are scalars. This can
    be changed with the 'returnrefs' or rr flag.
 
    With the above record !f! would match no fields, but !f!rr would match foo
    (which has a value of a hash ref)
 
    Options on KeyGroups:
       returnrefs, rr  - Return keys that have reference values (default:off)
       full, f         - Regex should match against full keys (recurse fully)
       depth=NUM,d=NUM - Only match keys at NUM depth (regex will match against
                         full keyspec)
       sort, s         - sort keyspecs lexically
 
 Help from: --help-keyspecs:
   KEY SPECS
    A key spec is short way of specifying a field with prefixes or regular
    expressions, it may also be nested into hashes and arrays. Use a '/' to nest
    into a hash and a '#NUM' to index into an array (i.e. #2)
 
    An example is in order, take a record like this:
 
      {"biz":["a","b","c"],"foo":{"bar 1":1},"zap":"blah1"}
      {"biz":["a","b","c"],"foo":{"bar 1":2},"zap":"blah2"}
      {"biz":["a","b","c"],"foo":{"bar 1":3},"zap":"blah3"}
 
    In this case a key spec of 'foo/bar 1' would have the values 1,2, and 3 in
    the respective records.
 
    Similarly, 'biz/#0' would have the value of 'a' for all 3 records
 
    You can also prefix key specs with '@' to engage the fuzzy matching logic
 
    Fuzzy matching works like this in order, first key to match wins
      1. Exact match ( eq )
      2. Prefix match ( m/^/ )
      3. Match anywehre in the key (m//)