Explain . . .
0. The solver is implicit-recursive
implicit: in the sense that it solves y given Φ(x,y)=0.
recursive: as it runs on the vectorized discrete x.
1. The low limit is searched first on a practical low range.
2. The upper limit is searched over a generously populated
range, starting from the low limit. You may attempt otherwise.
3. steps [1, 2] are trial/error.
4. Once all solutions are found, isolate for user to explore.
5. The model document is unique for the given Φ(x,y)
6. For some convenient value(s) ... userSolve(x)
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