= Solution
The <representation invariant> is $N\geq0$, `rows.length == N`, and every `rows(i)` is a nonnull finite <list> of entries $(j,x)$ satisfying
$$
\boxed{0\leq j<N,\quad x\ne0.0,\quad\text{column indices strictly increase along each row}.}
$$
Strict increase excludes repeated columns. The <abstraction function> gives $A_{ij}=x$ when the unique entry $(j,x)$ occurs in `rows(i)`, and $A_{ij}=0$ otherwise. Thus omitted positions represent zero; omission does not mean an unknown <matrix> entry.
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