[Retros] Forced e.p.: FIDE Laws of Chess

[Valery Liskovets]

Dear retro-friends,

Recently I've paid my attention to a minor but curious novelty in
the latest FIDE Laws of Chess 2004/05 (thanks to Otto!) concerning
e.p. - the last sentence in Article 3.7.d:
  "A pawn attacking a square crossed by an opponent's pawn which
  has advanced two squares in one move from its original square may
  capture this opponent's pawn as though the latter had been moved
  only one square. This capture is only legal on the move following
  this advance and is called an 'en passant' capture. This move must
  be made in the event that no other legal move is possible."

This last assertion was not present (although was always implied
of course) in the previous issues of the Laws (at least in the ones
available in the RA-Corner). It would be interesting to learn by
which concrete reasons it was added to the Laws.

Quite redundant for practical game, this explicit claim looks
as a ready theme for composition: a direct mate problem that
is solvable only due to Black's obligation to capture e.p., i.e. at
some moment in the solution, when bK is not under check, an
e.p. capture is the only legal Bl's move (neutral or weakening,
but anyway, Black is unhappy). This theme (or at least a special
technical element), fairly modest per se, which I suggest to call
"Forced en passant capture" (or "Forced e.p." for short), can be
easily implemented in two-mover with only 6 pieces; e.g.:
 A. W:Kg6,Qc7,Rd3,Pe2; Bl:Ke6,Pd4; #2 (1.e4 dxe3ep(!) 2.Qd6#).
However, presently I don't remember any published ordinary
problem with such a rare event in the solution, maybe unintentional.

What about retro? First of all, this spectacular element can be
naturally used in the "Who moves?" (="Whose move?") form
(where W mates Bl) combined with "En passant as key". I know
several such striking problems composed by W.Keym in 1966-8.
Here is one of them enriched by castling (1968, P1011955):
 B. W:Ke1,Rd2,Rh1,Be2,Bg5,Sd5,Sf2,Pa4,Pb3,Pb5,Pb7,Pc4,Pc6,Pd3;
    Bl:Kc1,Rb1,Ba1,Pb2,Pb4; #1 (1.0-0#?? 0... bxa3ep(!) 1.0-0#).

Are there other examples? In the PDB I've managed to find very few
ones. Here is the earliest problem among them, by B.R.Gioebel
(1931, P0004423):
 C. W:Ke4,Rc5,Sd2,Pa2,Pa5,Pb4,Pb5,Pc3,Pc6,Pd5; Bl:Ka4,Pa3,Pc4,Pc7;
    #2 (0... cxb3ep(!) 1.axb3+ Kxa5 2.Sc4#).

Besides, there exist several implementations in simple retractors.
The oldest one seems to belong to T.R.Dawson (1920, P0001850):
 D. W:Kf2,Rg3,Bf7,Sf5,Ph4; Bl:Kh5,Pf3,Pg4,Pg6; -1&#2
    (-1.h2-h4, 1.h4 gxh3ep(!) 2.Bxg6#).

Forced e.p. can be fruitfully combined with controversial genres
adding some extra paradoxicality to the latter: reversing colors.
In particular, for pRA in combination with "Who moves?" this was
done by W.Keym (1967, Heidelberger Tagblatt; missing in the PDB):
 E. W:Kb4,Qe8,Rc3,Rg3,Be2,Bh4,Se1,Se6,Pc5,Pd4,Pd6,Pe5,Pe7,Pf4,
    Pf6,Pg5; Bl:Ke4,Pc4,Pd5,Pe3,Pf5,Pg4; #1 (1.Rcxe3?? 1.Rgxe3#??
    0... cxd3ep(!) 1.Bxd3# or 0... gxf3ep(!) 1.Bxf3#).

The first such problem combining "A posteriori" with "Who moves?"
is mine published in Die Schwalbe (2004, P1066685):
 F. W:Ke1,Qf4,Ra1,Be2,Bg3,Se5,Pa3,Pd2,Pf2,Pf6,Pg5,Pg7,Ph4,Ph5;
    Bl:Kg1,Bh1,Pf5,Pg2,Pg4; #1 AP (1.0-0-0#??, 1.Bh2#??
    0... gxh3ep(!) 1.0-0-0#! 1.Bh2#??, 1.Sf3#?? - AP-illegal).

This auxiliary theme is also applicable to self-mate (retro-)problems.
But I don't know whether it is productive for combinations with
other retro-ideas.

Valery Liskovets
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A| _ : _ : _ : _ : | VL, 2007
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B| _ : _ : _ : _ : | W.Keym, 1968
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C| _ : _ : _ : _ : | B.R.Gioebel, 1931
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D| _ : _ : _ : _ : | T.R.Dawson, 1920
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E| _ : _ : Q : _ : | W.Keym, 1967
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F| _ : _ : _ : _ : | VL, 2004
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