Support layered constraints

[smartalecH]

I vaguely recall discussing with @lxvm and @stevengj ways to incorporate constraints between two stacked layers (e.g. such that TO doesn't yield "floating islands" or properly handles the run length between the top and bottom). One approach involves additional constraints and projections described in this paper.

But I believe during our discussion, we surfaced other approaches? Can we document those here? Are they compatible with SSP out of the box, and have they been implemented here?

[lxvm]

We have a simple way of imposing the partial etch constraints that works with SSP but we are still figuring out how to make it work well with length scale constraints

[smartalecH]

figuring out how to make it work well with length scale constraints

What's the current challenge? Do the two constraints compete?

[lxvm]

We often want features of different length scales in the two layers, but the length constraints are only meant to handle one length scale at a time. We have some ideas for how to adapt them, but haven't tried them out yet