Minor issue: The above three functions should really be map-over-... functions to both
avoid consing and be consistent with that convention throughout the rest of CLIM. | SWM

=> sheet-delta-transformation sheet ancestor [Generic Function]
Returns a transformation that is the composition of all of the sheet transformations between
the sheets sheet and ancestor. If ancestor is nil, sheet-delta-transformation will return the
transformation to the root of the sheet hierarchy. If ancestor is not an ancestor of sheet, the
sheet-is-not-ancestor error will be signalled.

The computation of the delta transformation is likely to be cached.

=> sheet-allocated-region sheet child [Generic Function]
Returns the visible region of the sheet child in the sheet sheet's coordinate system. If child is
occluded by any of its siblings, those siblings' regions are subtracted (using region-difference)
from child's actual region.

7.3.2 Sheet Geometry Classes

Each of the following implements the sheet geometry protocol in a different manner, according
to the sheet's requirements.

=> sheet-identity-transformation-mixin [Class]
This class is mixed into sheet classes whose coordinate system is identical to that of its parent.

=> sheet-translation-mixin [Class]
This class is mixed into sheet classes whose coordinate system is related to that of its parent by
a simple translation.

=> sheet-y-inverting-transformation-mixin [Class]
This class is mixed into sheet classes whose coordinate system is related to that of its parent by
inverting the y coordinate system, and optionally translating by some amount in x and y.

=> sheet-transformation-mixin [Class]
This class is mixed into sheet classes whose coordinate system is related to that of its parent by