== should format an + b selectors ==

:nth-child(1n+0) {}
:nth-child(n) {}
:nth-child(n+0) {}
:nth-child(-n+0) {}
:nth-child(-n) {}
:nth-child(N) {}
:nth-child(+n+3) {}
:nth-child(n+3) {}
:nth-child( +n + 3 ) {}
:nth-child(  N- 10) {}
:nth-child(n-10) {}
:nth-child(n- 10) {}

[expect]
:nth-child(n) {}

:nth-child(n) {}

:nth-child(n) {}

:nth-child(-n) {}

:nth-child(-n) {}

:nth-child(n) {}

:nth-child(n + 3) {}

:nth-child(n + 3) {}

:nth-child(n + 3) {}

:nth-child(n - 10) {}

:nth-child(n - 10) {}

:nth-child(n - 10) {}