Support combining markers?

I'm trying to write a PEP 621 consumer, and I'm having trouble with the optional-dependencies item. The issue is that I could potentially need to combine markers, and I don't see a particularly reliable way of doing so. Consider:

[project.optional-dependencies]
test = [
  "pytest < 5.0.0",
  "pytest-cov[all]",
  "pywin32; os_name == 'nt'"
]

To construct Requires-Dist from this, I need to add a dependency pywin32; os_name == 'nt' and extra == 'test'.

The best way I can see of doing this is

req = Requirement(req_str)
if req.marker is None:
    req.marker = Marker(f"extra == '{extra_name}'")
else:
    req.marker = Marker(f"({req.marker}) and extra == '{extra_name}'")

requires_dist.append(str(req))

Is that the best way of doing this? It would be extremely useful in this situation if markers supported & and | operations, to avoid the error-prone conversion to and from strings.

Start by reading the Requirement and Marker entry points referenced in the issue, then inspect how marker strings are parsed and rendered. Determine the intended semantics and API for combining markers, including precedence and parentheses, and validate the design against the PEP 621 optional-dependencies example.