Function parallel_evaluate 

pub fn parallel_evaluate(
    arena: &DagArena,
    chunks: Vec<Vec<DagNodeId>>,
    variables: &[f64],
) -> f64

Solves and evaluates a set of expression-leaf chunks in parallel.

Convenience wrapper: if you already hold an Arc<DagArena>, call parallel_evaluate_shared directly to avoid even the one-shot arena.clone() here. For callers that only have a borrow, this wraps once and dispatches.