Maybe this is a boring question, but I cannot figure it out. Because every expression has a `Head`, and `Head[1]` is `Integer`, and `Head[Integer]` is `Symbol`. Therefore, `1` should somehow represented as `Symbol["Integer"][1]` or something similar. However, `Depth[1]` is one which means `1` should be presented as `Symbol["Integer..."]`, not as a expression of depth 2, such as `Symbol["Integer"][1]`. 

What is correct representation of `1`?