How do I create a flat Y-blend ("chamfer" like)

[NiklasG (Niklas)]

How would I create such a Y blend? It should be like a chamfer surface but around a corner. Here is a picture:

With the corresponding Rhino8 file:
ChamferProblem.3dm (67.7 KB)

[Michael_Meyer]

_Patch: https://docs.mcneel.com/rhino/8/help/en-us/index.htm#commands/patch.htm