Class: backup_optigroup  - X-Ref

Instantiable class representing one optigroup element for conditional branching

Objects of this class are internally nested elements, so they support having both
final elements and children (more nested elements) and are able to have one source
and all the stuff supported by nested elements. Their main differences are:

- Support for conditional execution, using simple equality checks with outer values.
- Don't have representation in the hierarchy, so:
- Their level is the level of the parent of their enclosing optigroup.
- Act as one "path bridge" when looking for parent path values
- They don't support attributes

Their main use is to allow conditional branching, basically for optional submodules
like question types, assignment subtypes... where different subtrees of information
must be exported. It's correct to assume that each submodule will define its own
optigroup_element for backup purposes.