I would like to add my observations to this ongoing debate in EJP on après-coup, particularly its relationship to other psychoanalytic concepts, especially those that seem most closely related and to occupy the same ground. My reflections, in fact, being limited here, aim to stimulate further questions rather than answers, a path to follow rather than the destination.
Après-coup concerns temporality, and raises the question of whether there is a relationship, and what it is, between present and future, but also between future and past, inverting the irreversibility according to the presumed common logic of the temporal flow of events.
But a special defence called “undoing” (retroactive annulment)—while it also concerns temporality—also introduces the possibility of subtracting from a past event not just signification and influence, but its very existence; that is to say, it concerns the possibility of making the event something that did not happen, as with patients suffering from an obsessive neurotic disorder.
So undoing refers to the more general concept of defence, and along the way we discover how it is known, even if at times we don’t notice, that the defence is not a crusade (it does not mobilize an army of whites against blacks or greens etc.), but is itself kneaded with drive, that is to say, it is largely made up of the very material it must contrast. As Laplanche and Pontalis define it, defence is “marked and permeated by the drive”, that is, by the unconscious motivations it seems to oppose, and by the compromises reached under the impetus of other impulses or aims of the ego.
If we consider temporality in relation to psychic reality, we are referring to the unconscious (where, as we know, there exists a time different from that of the conscious). But we also observe a concrete external reality, of which we are conscious, with determined events: on the one hand those that precede a given time, and on the other those that follow it. It should be borne in mind, however, that for each of the facts perceived consciously, there corresponds an unconscious event whose memory trace is activated or reactivated on the basis of the dynamics of the motivations, and thus of the drives at play with their psychic presentations, and even earlier with the psychic representatives of these.
Clinically, we observe phenomena that are the effect of the colliding and reshaping of opposing instinctual impulses.
The above-mentioned conscious actions correspond to consciously assumed behaviors in the defence, and are marked by instinctual impulses and characteristics thanks to which the subject, orienting himself on the basis of what are considered valid conscious representations, seeks the satisfaction of certain actually unconscious motivations (instinctual impulses), or even, in the defence, the undoing of the same, going even against the logic of current reality. This happens within an overall psychic economy aimed at the safeguarding (from time to time, and according to the different phases of psychoanalytic theorizing or the perspective in which we are placed) of the Ego with its efforts to maintain its integrity and an overall psychic equilibrium, within the framework of the principle of constance, the pleasure principle and the principle of reality, or the conflict of drives against drives of a different if not opposed sign. We know that the defence is at least for the most part unconscious and as such mostly acts, thus also addressing, among other things, the need to differentiate between observable symptomatic behaviors like neuroses—which can be categorized as defences involving the conscious participation of the subject who nevertheless ignores the true unconscious motivation, and which are thus perceived and employed by the subject—and those behaviors instead referable to unobservable but inferable defences.
It suffices for our purposes here to have declared that the defence does exist, and that it exemplifies, even in its specificity, a basis for the functioning of the psychic apparatus, thus showing the possible coexistence within it of contradictions and apparent deficiencies of meaning, and vice versa of an activity capable of synthesis to reach complex psychic goals that cannot be defined in a linear manner.
What is important now is to question how these three functionings—of the defence, of après-coup, and of retroactive annulment—manifest themselves? And are there any points of contact, do they share something?
One point seems relevant here: that there exists a condition that initiates mechanisms capable of promoting, starting from a discrete experience, the search for something aimed at finding a perceptual identity (such an identity can also concern negative experiences), or a defence from something whose memory trace can be re-proposed and re-activated, despite the will of the subject obviously, along a path that seems for certain aspects common to what is based on research, and to what is based on the refusal to achieve a perceptual identity. A path, that is, aiming at the search for, or the avoidance of, a perceptual identity between the current experiences and the originary experiences of first pleasure or first pain respectively—concepts that Freud tackled at the beginning of his reflections when drafting his Project for a Scientific Psychology. And, as we know (primarily thanks to the reconstruction of Freudian theory left to us by Jacques Lacan), it is a work wherein—in the relationship of primary importance with the significant adult (the mother, the lost object, the symbol of the Nebenmensch), and essentially thanks to language—das Ding (the Thing) is identified, that is, neuron a, the matricial mold of everything, with neuron b its predicate.
Après-coup and undoing, or retroactive annulment, seemingly two antipodal entities—the first hesitating over the fulfillment of something, an obstacle that, working backwards from the past or on the past, thus becomes decisive for the future event, the other instead turned towards a cancellation not only of the meaning, but of the very existence of a fact—reveal themselves surprisingly as two sides of the same coin, their connection so far never grasped, like the translation into psychic events and corresponding observable behaviors of a powerful activity that revolves around the attainment  or the cancellation of a perceptual identity. This identity, in fact, is essential not just because something—the disturbance—can be realized (and in a dynamic that obviously always includes the defence besides the drives and traumas) with all the inherent implications, but also because, without having to bring onto the scene the magic and subversion of common sense, something can be not only denied, but completely eliminated, by defusing what makes it an uncancelled psychic fact, and that in the psychic scenario it is possible to cancel only in the analytic process through the resolution of its link with the unconscious motivation, both in the treatment of hysterical and obsessive disorders.
But the perhaps aporetic conceptual core of the question lies in the fact that, at least as far as its objective is concerned, in my opinion it is exactly the same thing, namely an avant-coup, that is the perceptual identity, the underlying mechanism implied in both the après-coup and the undoing, or retroactive cancellation. And it seems legitimate to consider that the perceptual identity that, with opposing purposes (discovery or avoidance), is in question, is not necessarily attributable simply to a proto-experience, but is limitlessly multiplied to other experiences during the course of existence. As we know, within the signifiers-signified chain, any event can be traumatic and thus constitute an avant-coup of après-coup and vice versa. In the case of undoing, the avant-coup would reside in the ambivalent conflict underlying the obsessional neurosis, and in the mnestic trace, in its correlated mnestic image that was removed, and that is in this discussion the object of the perceptual identity that the undoing, the retroactive cancellation, rejects.
Any event, be it spontaneous or prompted by psychoanalytic treatment, is in any case capable of initiating a modification of the entire system, without any single predefined direction of the arrow of time, and without the need of finding any predefined order to make sense of events, and if anything, with the task of knowing how to recognize a valid one every time.
In this sense, both Freud’s expression “diphasic” applied to obsessive symptomatology (and in my opinion also applicable to hysteria), as well as the singular referring to the psychic event whether avant-coup or après-coup, should both be reconsidered: the disorder is n-phasic and the avant-coup or après-coup are words which, like adjectives or adverbs, indicate complex, plural events that can actually include a multiplicity of factors and are, in any case, in psychoanalytic terms, involved in a multiple function, and overdetermined.
 certainly involuntary and triggered in its pathogenicity by the greater maturity and psychic awareness of the past event in the case of Emma’s neurosis.