This helper was drafted for the Job / JobPlanning and Scheduler
interface in 2013, but seemingly not yet put into action. While
in the original use case, we have a genuine measuerment for the
tree depth (given by the depth of the processing stack), in other
use cases we want to use to offset embedded within the indicator
itself for keeping track of the depth. Thus I add a second
mark operation, which usess the current offset to set a new
reference level. This has the consequence that the offset
has now to reflect the new reference point immediately
