Lattice expression
chain(n) is the subgroup lattice of Cpn,
divisors(n) that of Cn, and
M(n) the height-two lattice with n atoms.
Combine with * for products, + for ordinal sums,
| for horizontal joins.
Lattice as JSON
covers lists covering pairs, action one permutation
of element indices per generator of the acting group. Everything else is
derived. Validation names the pair that fails.
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Covers of the current system, computed on demand. The full lattice of
transfer systems is never built, so this stays responsive however large it is.