Current depth (starts at 0, continues with 1, 2...). Only made of integer values superior or equal to 0.
Remark: Whenever altering the depth during a generate, please make sure to ALWAYS
reset it to its original value before you leave the generate. Otherwise the execution
will imply side-effects that will potentially impact the following runs and make replay
of the issue barely impossible.
Instance of depth, can be used to alter the depth perceived by an arbitrary or to bias your own arbitraries based on the current depth