  FRB 190520B, a repeating FRB near-twin of FRB 121102, was discovered
\citep{N21} to have a dispersion measure excess over the intergalactic and
Galactic contributions of about 900 pc-cm$^{-3}$, attributable to its host
galaxy or near-source environment. This excess varies on a time scale of $\sim
30\,$y and might be explained by a supernova remnant no more than a few decades
old. A magnetic field in equipartition with the remnant's expansion would be
${\cal O}(1\,\text{G})$. I suggest the use of baseband data to measure very
large rotation measures.
