Exercise 1: Crib-Dragging Worked Example — Possible Solution
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Ciphertext (12 letters, 1-indexed):
  1:R  2:W  3:E  4:F  5:T  6:B  7:E  8:R  9:X  10:T  11:Q  12:Y

Crib: W E T T E R (6 letters)

OFFSET 1 -> compares against ciphertext positions 1-6: R W E F T B
  crib:      W  E  T  T  E  R
  cipher:    R  W  E  F  T  B
  matches?   no no no no no no
  Result: NO self-matches -> offset 1 SURVIVES (not eliminated)

OFFSET 4 -> compares against ciphertext positions 4-9: F T B E R X
  crib:      W  E  T  T  E  R
  cipher:    F  T  B  E  R  X
  matches?   no no no no no no
  Result: NO self-matches -> offset 4 SURVIVES (not eliminated)

OFFSET 7 -> compares against ciphertext positions 7-12: E R X T Q Y
  crib:      W  E  T  T  E  R
  cipher:    E  R  X  T  Q  Y
  matches?   no no no YES(T=T) no no
  Result: crib's 4th letter (T) lines up with ciphertext position 10,
  which is also T -> a letter would have to map to itself, which
  Enigma's reflector makes impossible -> offset 7 is ELIMINATED.

Summary: offsets 1 and 4 survive as possible alignments; offset 7 is
ruled out entirely.

WHY THIS WORKS AS AN ANSWER
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This applies the chapter's own three-step crib-dragging procedure
exactly as described: compare the crib letter-by-letter against the
ciphertext at each candidate offset, and discard any offset where a
letter matches itself. Offset 7 is eliminated with zero knowledge of
the actual daily key — pure structural reasoning from the reflector's
"never itself" guarantee (Chapter 3) is enough on its own. Note that
two offsets (1 and 4) both survive this first pass, which is realistic
and matches the chapter's own description of the technique as
narrowing the candidates down, not always pinpointing a single unique
answer by itself — further testing (additional cribs, or passing
surviving candidates to the Bombe) would be needed to distinguish
between them.