I thought Freud had mentioned "in tandem", as Nabokov did, while describing one of Ada and Van's intercourses. However, the Latin word employed by Freud is "a tergo" Cf.  From "The History of an Infantile Neurosis"  SE vol. XVII, The Dream and Primal Scene (p. 37) (The Wolf Man):

"It harmonizes with our assumption that it was a hot summer's day, if we suppose that his parents had retired, half undressed, for an afternoon siesta. When he woke up, he witnessed a coitus a tergo (from behind), three times repeated." 
footnote: "Why three times? He suddenly one day produced the statement that I had discovered this detail by interpretation. This was not the case...in his usual way he passed it off on to me, and by this projection tried to may it seem more trustworthy."

 

While checking the clinical material about the Wol Man I came accross two interesting minute details (related to his childhood, the seduction by his sister, famous primal scene, English governesses and Russian heritage). Freud is a great writer, his case-histories can read like novels and often, today, I felt I was wandering in secluded spot in Ardis.

1. "Once he was running after a beutiful big butterfly, with striped yellow wings which ended in points, in the hope of catching it (It was no doubt a swallow-tail) footnote: [Schwalbenschwanz]" Here and at the beginning of section VIII below, the edtions before 1924 read 'Admiral" (SE XVII, p 18)"

2. "He had repeatedly specified before that his sister had shot herself; but he was now obliged to make a correction and say that she had taken poison. The poet, however, had been shot in a duel.  Footnote: No doubt Pushkin. (SE XVII, p.23)**

 

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* A Freudian contribution about  coital "a tergo," primal scenes and infantile birth-sexual theories: Cf. Sigmund Freud SE, vol. XVI,p.355: "In childhood the female genitals and the anus are regarded as a single area - "the bottom" (in accordance with the infantile cloaca theory)":
More information about it on section two of "Three Essays on..." (1905d); in "Introductory Lectures," n.XXIII; in sections V & VIII from the Wolf Man (1918b) 

Der Wolfsmann In January 1910, Pankejeff's physician brought him to Vienna to have treatment with Freud. Pankejeff and Freud met with each other many times between February 1910 and July 1914, and a few times thereafter, including a brief psychoanalysis in 1919. Pankejeff's "nervous problems" included his inability to have bowel movements without the assistance of an enema, as well as debilitating depression. He also felt like there was a veil cutting him off from the world. Initially, according to Freud, Pankejeff resisted opening up to full analysis, until Freud gave him a year deadline for analysis, prompting Pankejeff to give up his resistances.
Freud's first publication on the "Wolf Man" was "From the History of an Infantile Neurosis" (Aus der Geschichte einer infantilen Neurose), written at the end of 1914 but not published until 1918. Freud's treatment of Pankejeff centered around a dream the latter had as a very young child, and described to Freud as such:
"I dreamt that it was night and that I was lying in bed. (My bed stood with its foot towards the window; in front of the window there was a row of old walnut trees. I know it was winter when I had the dream, and night-time.) Suddenly the window opened of its own accord, and I was terrified to see that some white wolves were sitting on the big walnut tree in front of the window. There were six or seven of them. The wolves were quite white, and looked more like foxes or sheep-dogs, for they had big tails like foxes and they had their ears pricked like dogs when they pay attention to something. In great terror, evidently of being eaten up by the wolves, I screamed and woke up. My nurse hurried to my bed, to see what had happened to me. It took quite a long while before I was convinced that it had only been a dream; I had had such a clear and life-like picture of the window opening and the wolves sitting on the tree. At last I grew quieter, felt as though I had escaped from some danger, and went to sleep again." (Freud 1918)
Freud's eventual analysis (along with Pankejeff's input) of the dream was that it was the result of Pankejeff having witnessed a "primal scene" — his parents having sex a tergo ("from behind" or "doggy style") — at a very young age. Later in the paper Freud posited the possibility that Pankejeff had instead witnessed copulation between animals, which was displaced to his parents.
Pankejeff's dream would play a major role in Freud's theory of psychosexual development, and along with Irma's injection (Freud's own dream, which launched dream analysis), it was one of the most important dreams for the developments of Freud's theories. Additionally, Pankejeff became the main case used by Freud to prove the validity of psychoanalysis. It was the first detailed case study not involving Freud analyzing himself which brought together the main aspects of catharsis, the unconscious, sexuality, and dream analysis put forward by Freud in his Studies on Hysteria (1895), The Interpretation of Dreams (1899), and his Three Essays on the Theory of Sexuality (1905).
Pankejeff would later publish his own work under Freud's given pseudonym, and would be in contact with Freudian disciples until his own death (undergoing analysis for six decades, despite Freud's pronouncement of his being "cured"), making him one of the longest-running famous patients in the history of psychoanalysis.
A few years after finishing psychoanalysis with Freud, Pankejeff developed a psychotic delusion. He was observed walking the streets staring at his reflection in a mirror, convinced that some sort of doctor had drilled a hole in his nose. Ruth Mack Brunswick, a Freudian, explained the delusion as displaced castration anxiety.
Criticism of Freud's interpretation: Critics, beginning with Otto Rank in 1926, have questioned the accuracy and efficacy of Freud's psychoanalytic treatment of Pankejeff.. Daniel Goleman, in the New York Times, wrote the following: Freud's key intervention with the Wolf Man rested on a nightmare in which he was lying in bed and saw some white wolves sitting on a tree in front of the open window. Freud deduced that the dream symbolized a trauma: that the Wolf Man, as a toddler, had witnessed his parents having intercourse. Freud's version of the supposed trauma, however, was contradicted by the Wolf Man himself, Sergej Pankejeff, in an interview with Karin Obholzer, a journalist who tracked him down in Vienna in the 1970s. Mr. Pankejeff saw Freud's interpretation of his dream as 'terribly far-fetched.' Mr. Pankejeff said, 'The whole thing is improbable,' since in families of his milieu young children slept in their nanny's bedroom, not with their parents.
Mr. Pankejeff also disputed Freud's claim that he had been cured, and said he resented being 'propaganda' and 'a showpiece for psychoanalysis.' Mr. Pankejeff said, 'That was the theory, that Freud had cured me 100 percent.' However, 'It's all false.' Maria Torok and Nicolas Abraham have also reinterpreted the Wolf Man's case (in The wolf man’s magic word, a cryptonymy), presenting their notion of "the crypt" and what they call “cryptonyms." They provide a different analysis of the case than Freud, whose conclusions they criticise. According to the authors, Pankejeff's statements hide other statements, while the actual content of his words can be illuminated by looking into his multi-lingual background. According to the authors, Pankejeff hid secrets concerning his older sister, and as the Wolf Man both wanted to forget and preserve these issues, he encrypted his older sister, as an idealised "other" in the heart of himself, and spoke these secrets out loud in a cryptic manner, through words hiding behind words, rebuses, wordplays etc. For example, in the Wolf Man's dream, where six or seven wolves were sitting in a tree outside his bedroom window, the expression "pack of six", a "sixter" = shiestorka: siestorka = sister, which gives the conclusion that his sister is placed in the centre of the trauma. The case forms a central part of the second chapter of Gilles Deleuze and Félix Guattari's A Thousand Plateaus, titled "One or Several Wolves?" In it, they repeat the accusation made in Anti-Oedipus that Freudian analysis is unduly reductive and that the unconscious is actually a "machinic assemblage". They argue that wolves are a case of the pack or multiplicity and that the dream was part of a schizoid experience. "The greatest triumph and cure Freud reported was the case of the Wolf man—Sergei Pankeev, who was crippled with depression and anxiety and phobia about wolves from childhood. Freud settled upon the traumatic effects of the so-called primal scene, when an infant witnesses his parents copulating; the Oedipus complex and the fear of castration; and the tripartition of the mind into id, ego, and superego. As is general knowledge, Freud made castration anxiety and the Oedipus complex not merely the cause of neurosis but the indispensable, universal cause of the formation of each individual’s superego, and thus the crucial factor in the repression and control of primitive impulses—and so in the preservation of civilization. Freud published the case in 1918 where he claimed to have cured Pankeev completely, freeing him of all of his fears and obsessions, however, the status of his cure is debatable. For nearly 70 years, Pankeev was in and out of analysis with Freud and his followers with his condition worsening, until Freud's death. In 1970s, an Austrian journalist, Karin Obholzer, found him and interviewed him at length. Pankejeff told her, in despair, “the whole thing looks like a catastrophe. I am in the same state as when I came to Freud, and Freud is no more.”