Making the assertions in AboutLet.fs pass

What did I learn?...

Confirmation of some of the things I'd guessed about let...
- it binds an identifier to a value
- the identifier does have a type, which is usually inferred
- you can use type annotations to explicitly provide the type,
	let (x:int) = 42

F# has all the normal .NET fundamental types (with some naming differences, e.g., float is the same as a C# double, float32 is a C# float)

Variables in F# are immutable by default ...trying to change the value after binding it causes a compiler error
You have to specifically declare it as mutable if you're going to change it...
	let mutable x = 100

This is how you modify the value of a variable (as long as it's mutable)...
	x <- 200

You can kind of modify mutable variables by re-using the name in another let statement.
(...something about 'shadowing' but I'm not sure what this is yet?)

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