AST node for the solve(eq, v) functional (parser: solve(expr = expr, v)).
eq is typed as _Expression, not [[_Equation]], so a named equation binding (h := x = 5) can be passed directly: after substitution expands h to _Equation(x, 5), eval pattern-matches on the concrete _Equation and delegates to solve. If eq is not an _Equation at eval time (e.g. a plain expression or an _EqualityCheck), the node stays symbolic -- only = builds a solvable relation, not ==.
A solution set is never a concrete _Value, so eval always answers Left:
no solution found / known -> Left(this) (stays symbolic)
exactly one solution -> Left(v = expr) (an _Equation)
several solutions -> Left([[v=e1, v=e2]]) (a row-vector _Matrix)
children exposes eq as a single child (binder v is excluded), so substitute/dependsOn/rebuild walk into eq -- if eq is a _Variable naming a definition, substitute replaces it with its body before eval runs.
Value parameters
eq
the equation expression (must resolve to an _Equation at eval time)
v
the solve variable (binder -- excluded from children)
