What is the POSTCSS equivalent for the Ampersand(&) commonly used in LESS, SASS and STYLUS?

Question

So with less I could do & to do a combination.

eg:

.baa {
  &.foo {
    opacity: 1;
  }
}

which would generate a rule .baa.foo

What plugins and syntax do we need for postcss to do the same thing?

Answer 1

For posterity,

we need this postcss plugin:

https://github.com/postcss/postcss-nested