Variable identity is not stable during tree expansion/compaction

When there is a variable on a concurrent execution and we compact the execution tree, then the variable is moved to the scope execution.

However, the implementation works in a way that recreates the variable instance on the scope execution and removes it on the concurrent execution. This has the problem that the corresponding variable instance is recreated and receives a new ID. Existing history entries for that variable can no longer be safely related to that variable instance.