> 2) RPN has a short syntax for them << a -> a 2 + >>

The syntax is slightly different. What it should be is this

<< -> a << a 2 + >> >>

The arrow introduces a binding form that captures the top n levels of the stack into n named local variables. (There's also a lower level implementation that forgoes the names in favor of a lookup-by-ordinal scheme that's a lot faster.)

The other notable difference from what you might expect is that the lexical environment is not captured:

Consider this function:

<< -> a << << a 2 + >> >> >>

What it returns is a function object that looks like this, except that 'a' is a local variable reference rather than the global variable reference it looks like:

<< a 2 + >>

Evaluating this results in an error, because while 'a' is represented as a local variable reference, the environment containing the local binding for 'a' is no longer available.