Gets or sets the stack.
Validation is a
recursive process. One Element validates each of its
child Elements. Each
of those in turn validate their children.
In the event that an inconsistency is found, it useful to know
how the current Element was arrived at.
This property allows you to examine the current state of the
stack including all parent StackNodes up to the root.
There may be apparent duplicates on the stack as sub-classes may
result in multiple calls to the Start function.
For this reason, for a stack trace, you will generally want to
use the GetStackTrace function.
However you may also find that you want to Peek at the top -
current - item for added detail when reporting errors.