(defn >!
  "puts a val into port. nil values are not allowed. Must be called
  inside a (go ...) block. Will park if no buffer space is available.
  Returns true unless port is already closed."
  [port val]
  (assert nil ">! used not in (go ...) block"))

(defn <!
  "takes a val from port. Must be called inside a (go ...) block. Will
  return nil if closed. Will park if nothing is available."
  [port]
  (assert nil "<! used not in (go ...) block"))

  "Asynchronously executes the body, returning immediately to the
  calling thread. Additionally, any visible calls to <!, >! and alt!/alts!
  channel operations within the body will block (if necessary) by
  'parking' the calling thread rather than tying up an OS thread (or
  the only JS thread when in ClojureScript). Upon completion of the
  operation, the body will be resumed.

  go blocks should not (either directly or indirectly) perform operations
  that may block indefinitely. Doing so risks depleting the fixed pool of
  go block threads, causing all go block processing to stop. This includes
  core.async blocking ops (those ending in !!) and other blocking IO.

  Returns a channel which will receive the result of the body when
  completed"