Submesh subdomain ID union

[connorjward]

It would be nice to support an interface to Submesh like:

submesh = Submesh(mesh, subdomain_id=(1, 2, 3))

where we allow subdomain_id to be a tuple of values and the returned submesh would be the union of these subdomain IDs.

To implement this we would likely have to construct a temporary label representing this union, call dm.filter(), then destroy the label.

[pbrubeck]

We could also support subdomain intersection. Potentially we could have an interface like this one:

submesh = Submesh(mesh, tdim-1, subdomain_id="1&2")

[connorjward]

I've figured out how to do this: use pyparsing

import pyparsing as pyp


obj = pyp.infix_notation(
    pyp.Word(pyp.alphanums + "_"),
    [
        ("&", 2, pyp.OpAssoc.LEFT),
        ("|", 2, pyp.OpAssoc.LEFT),
    ],
)

print(obj.parse_string("on_boundary").as_list())
print(obj.parse_string("2&3").as_list())
print(obj.parse_string("2|3").as_list())
print(obj.parse_string("1&2|3").as_list())
print(obj.parse_string("1|2&3").as_list())
print(obj.parse_string("(1|2)&3").as_list())

prints

['on_boundary']
[['2', '&', '3']]
[['2', '|', '3']]
[[['1', '&', '2'], '|', '3']]
[['1', '|', ['2', '&', '3']]]
[[['1', '|', '2'], '&', '3']]