The Children of the hggroup are a set of patches, one patch per contour level.
The patches are basically X/Y/Z lists of coordinates, with the Z originally set to be constant at the contour level. The transitions between the various regions of contouring are, if I recall correctly, done by sticking NaN in to the list of points where the breaks are.
After the utility routine constructs all of these patches, the contour() routine deliberately goes through and sets the Z coordinates for all of the patches to 0, destroying the original Z. Fortunately, the utility routine that constructed the layered patches also stored the patch level (Z coordinate) in the UserData field associated with the patch, so it is possible to determine which patch is which by looking at its UserData field.
The contour regions are sometimes quite irregular, especially if two regions with the same level touch each other: in such a case it is not certain that those two regions will be done through X1 NAN X2 NAN or if it will just build X1 X2 NAN.
