added references:
Discussion as an operad in spectra (in stable homotopy theory) is in
based on
Michael Ching, _Bar constructions for topological operads and the Goodwillie derivatives of the identity, Geom. Topol. , 9:833–933, 2005 (arXiv:math/0501429)
Michael Ching, Bar-cobar duality for operads in stable homotopy theory, Journal of Topology, 2012 (arXiv:1009.5034)
added three references, as per Harry’s request here. But it remains very incomplete
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