IPending

protocolsince v0.0-927 clojure.lang/IPendingEdit
implemented for Cycle Delay Iterate LazySeq Repeat

Source docstring:
Protocol for types which can have a deferred realization. Currently only
  implemented by Delay and LazySeq.
Source code @ clojurescript:src/main/cljs/cljs/core.cljs
(defprotocol IPending
  (^boolean -realized? [x]
    "Returns true if a value for x has been produced, false otherwise."))