I think I mostly got this, but just to test it, it would be like in Typescript where I might say:

    type Foo = { x: number; }
    type Bar = { x: number; y: number }
    type FooBar = Foo | Bar;
    function baz(x: FooBar) {
      if ('y' in x) {
        // compiler now knows x is a Bar
      }
    }

In this case, the variable `x` has a property that is determined by the compiler based on control flow. i.e. it isn't explicitly carried by the type of `x`.