Sketcher ConstrainInternalAlignment

Description
This constraint aligns lines and points to particular places of a complex sketcher element (there is just one "complex" element so far, the Ellipse).

For Ellipse and its Arc, it supports constraining lines to become major and minor diameters, and constraining points to positions of ellipse's foci.

The constraint requires a lot of effort to use in the way other constraints are. It is hidden in the menu, and not exposed on any toolbars by default. There is a helper tool called Show/Hide Internal Geometry which is exposed on workbenches' toolbars and aimed to completely remove the need to invoke the constraint manually.

Operation on Ellipse

 * 1) Select elements to be aligned and an ellipse. The ellipse must be selected last. Accepted are up to two lines and up to two points.
 * 2) Invoke the constraint by picking the menu item (Sketch → Sketcher constraints → Constrain InternalAlignment).

The first line that was selected gets aligned to become ellipse's major diameter (but if it is not occupied already by another line, otherwise it will become minor diameter). The second line is aligned to become minor radius. The lines are automatically switched to construction.

Likewise, the first point is constrained to become the first unoccupied focus, and the second point goes to the other focus.

Scripting
Remarks:
 * Sketch is a sketch object.
 * Number 1 in the focus calls stands for starting point of a point element (it is ignored).

Version
Introduced in FreeCAD v0.15.4309