Prepare a conceptual domain model (class diagram) for a graphical document editor.

Assume that a document consists of several sheets. Each sheet contains drawing objects,

including text, geometrical objects, and groups. A group is simply a set of drawing objects,

possibly including other groups. A group must contain at least two drawing objects. A drawing

object can be a direct member of at most one group. Geometrical objects include circles,

ellipses, rectangles, lines, and squares.