We see in this notebook the use of `let` blocks, which is not typical with `Pluto`. As `Pluto` is reactive – meaning changes in a variable propagate automatically to variables which reference the changed one – a variable can only be used *once* per notebook at the top level. The `let` block, like a function body, introduces a separate scope for the binding so `Pluto` doesn't incorporate the binding in its reactive model. This is necessary as we have more than one function named `f`. This is unlike `begin` blocks, which are quite typical in `Pluto`. The `begin` blocks allow one or more commands to occur in a cell, as the design of `Pluto` is one object per cell.
