I just noticed that https://docs.python.org/3/library/string.html#formatspec links to the "integer" definition in the main Python grammar for the permitted format of numeric fields.

This isn't accurate:

```
>>> format(10e4, ",.2f")
'100,000.00'
>>> format(10e4, ",.0x2f")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
>>> format(10e4, ",.2_2f")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
```

Instead, the `width` and `precision` subfields should be defined as "digit+"