Usage  
- Install or run the script from Text Editor  
- Select one vertex on the corner of two connected edges (so be in vertex selection mode)  
- go to object mode to let the script know what vertex you want to operate on  
- back into editmode, hit Check Vertex. The fillet should start drawing  
- adjust the settings to satisfy  
- hold shift and press numpad_plus / numpad_minus to adjust the segments  range(2...64)
- hit Enter to apply the fillet, or Esc to stop drawing.  

Things to avoid  
- avoid pressing Check vertex if the fillet is already drawing.  
- don't try this on geometry that doesn't have all its transforms applied (rotation,location,scale)
- telling me it doesn't work, without an error message or .blend case file.

Objective
fillet the selected vertex of a profile. the max fillet radius is reached at the shortest delta of surrounding verts.

warning. using "ctrl+numpad +/-" to change the current selection of vertices will result in no changes. no fillet.

Milestone 1
[x] get selected vertex index.
[x] get indices of attached verts.
[x] get their lengths, return shortest.
[x] shortest length is max fillet radius.

Milestone 2
[x] draw gl bevel line from both edges (bev_edge1, bev_edge2)
[x] find center of bevel line
[x] draw centre guide line.
[x] call new found point 'fillet_centre"
[x] calculate verts "KAPPA"
[x] calculate verts "TRIG"

Milestone 3
[x] draw faux vertices
[x] draw opengl filleted line.
[x] allow view rotate, zoom
[x] allow "shift+numpad +/-" to define segment numbers.
[x] "esc/rclick" to cancel.
[x] sliders update in realtime
[x] "enter to accept", and make real.
[x] checks revision, uses code according to your blender release.
[x] cleanup
[x] create negative / positive switch.
[ ] make shift+rightlick, draw manipulation line to mouse cursor.
[ ] make faces tri, quad (quad option only available with appropriate num_verts)
[ ] user must apply all transforms, or matrix * vector (this i don't understand yet)