Create framework allowing for sub modes within editing.

Adrian spoke about being able to draw vertical/horizontal lines while creating
a geometry. I think the topic goes beyond to the point that, while editing
geometries, it is necessary to combine different techniques, thus the
selection of a feature creation/modification tool should set a "mode", in
order that other context sensitive "modifiers" can be used.
Examples:

• being editing a line and set an "ortho" mode which only allows horizontal
  and vertical lines, or even enabling the drawing of line segments orthogonal
  to another one from another feature.
• being editing a line and set the edition mode temporarily parallell to
  another, existing one.
• while adding vertices to a line, draw an arc and then continue
  adding straight segments.

of course there are much more possible examples. AutoCAD is the most feature
rich app I know on this regard.