int factor = 2;
    assert([2, 3].map!(a => a * factor).array == [4, 6]);

So what you're passing is actually purely a function symbol. The way that it gets the stack reference to the surrounding function is that it's actually a nested function. And the way that `map` gets the stack reference to pass to the lambda is that, effectively, that instance of `map` is also a nested function of the calling function.

That's also why you cannot pass a lambda as a template parameter to a class method in D: it already has a context parameter, ie. the class reference.

In Neat, the value of the lambda is the stackframe reference, and it's just passed as a regular parameter:

    int factor = 2;
    assert([2, 3].map(a => a * factor).array == [4, 6]);