Lag label#228
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danshapero
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May 15, 2026
| return enew | ||
| else: | ||
| return ufl_replace(e, cmapping) | ||
| new_terms = [] |
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I had a bit of help from Claude for this code path. The alternate path below (not a LabelledForm) was straightforward.
pbrubeck
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May 15, 2026
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| for var in extract_type(e, Variable): | ||
| if var.ufl_operands[1] is lag_label: | ||
| cmapping.setdefault(var, var) |
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This should go at the top of the function, and the rest of the diff could go away
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I just noticed that #227 might potentially be useful for similar purposes.
I think it's fine to support both modes. |
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Resolves #221. This PR adds a function
lagwhich labels a single expression in a form for holding back to the start of a timestep. As @pbrubeck thought, implementing it amounted to modifyingirksome.tools.replace. I added a test based on lagging the conductivity in the Stefan problem. Without lagging, that fails because of the conditional.In addition to giving you an escape hatch, you can use lagging to define nonlinear preconditioners. I have a demo of this for the Allen-Cahn equation coded up just using petsc4py which I think is pretty compelling. @JHopeCollins and I are working on getting AuxiliaryOperatorSNES merged into Firedrake: firedrakeproject/firedrake#5119. Once that's merged I can write an Irksome demo showing that off, either in this PR or a separate one.