JavaScript anonymous function immediate invocation/execution (expression vs. declaration)

Possible Duplicates:
What is the difference between a function expression vs declaration in JavaScript?
Explain JavaScript's encapsulated anonymous function syntax

Why this:

(function () {
    //code
}());

and this:

var f = function () {
    //code
}();

works, while this:

function () {
    //code
}();

does not? It looks exactly the same - anonymous function defined, and immediately called. Can someone make a quotation from the JavaScript/ECMAScript standard which explains that?

UPDATE: Thanks for the answers everyone! So it's about function expression vs. function declaration. See this Stack Overflow answer, ECMAScript standard section 13, and this great article: Named function expressions demystified.

To recap answers:

1. The first snippet is interpreted as an expression because the grouping operator, (), is applied - see ECMAScript standard section 11.1.6.

2. In the second snippet, the function is interpreted as an expression because it's on the right-hand part of assignment operator, =.

3. The third snippet doesn't have anything which allows the interpreter to read the function as an expression, so it's considered a declaration, which is invalid without an identifier (Gecko lets it pass however, but it chokes on following () grouping operator (as it thinks) applied to nothing).