While watching a stream by @awygle I noticed Fragment.get() has a while True loop to chain elaborate calls. This could be improved by detecting possible loops in elaboration, in the event that some elaboration-time metaprogramming has a bug that causes a cycle.

For example, the following test case will never complete elaborating:

from nmigen import *
from nmigen.back import verilog

class Example(Elaboratable):
    def elaborate(self, platform):
        return self

e = Example()
print(verilog.convert(e))