internal class CombinedContext(
    private val left: CoroutineContext,
    private val element: Element
) : CoroutineContext, Serializable {

    override fun <E : Element> get(key: Key<E>): E? {
        var cur = this
        while (true) {
            cur.element[key]?.let { return it }
            val next = cur.left
            if (next is CombinedContext) {
                cur = next
            } else {
                return next[key]
            }
        }
    }

    public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
        operation(left.fold(initial, operation), element)

    public override fun minusKey(key: Key<*>): CoroutineContext {
        element[key]?.let { return left }
        val newLeft = left.minusKey(key)
        return when {
            newLeft === left -> this
            newLeft === EmptyCoroutineContext -> element
            else -> CombinedContext(newLeft, element)
        }
    }

    private fun size(): Int {
        var cur = this
        var size = 2
        while (true) {
            cur = cur.left as? CombinedContext ?: return size
            size++
        }
    }

    private fun contains(element: Element): Boolean =
        get(element.key) == element
}

问题

 public override fun minusKey(key: Key<*>): CoroutineContext {
        element[key]?.let { return left }
        val newLeft = left.minusKey(key)
        return when {
            newLeft === left -> this
            newLeft === EmptyCoroutineContext -> element
            else -> CombinedContext(newLeft, element)
        }
    }

element[key]?.let { return left }这句代码语义是啥意思？
return left 指的是 return 到 let 这层，还是 minusKey这一层。