Function evaluation 

pub fn evaluation(
    site: &mut EvalSite<'_>,
    value: &Value,
    evaluate: &Evaluate,
) -> Value

Reduce an expression tree with an evaluator, innermost first.

An implicit list is reduced a member at a time as its members close and again as a whole once it is complete, so a value the evaluator has already produced is returned untouched rather than reduced twice. That record is per-thread and is cleared with the expression arena.