Easy way to get double cornered edges

[Jollywatt]

This kind of edge seems quite common:

This is possible to do, but you have to write the coordinates of both corners manually (e.g., with coordinate expressions).

It may be worth adding an easier way, either by:

• generalising the corner option of edge(), e.g., to accept an array (left, right) or (right, left).
• adding a custom coordinate expression?