Function explain 

pub fn explain<C>(
    node: &Value,
    ctx: &mut C,
    resolve: Resolve<'_, C>,
    options: Option<&EvalOptions<'_, C>>,
) -> Result<TraceNode, ExpressionError>

Evaluate an expression tree and return the verdict tree — the regex-debugger view: every evaluated node, its own result, and (for ordered comparisons) the identities it compared. The root’s value is exactly what evaluate returns for the same inputs; the conformance suite runs every vector through both and requires agreement, so the two entry points cannot drift. Kept separate from evaluate so the hot path never pays for trace allocation. Errors propagate exactly as in evaluate — a failed evaluation yields an error, not a partial trace.