The Expr
========
       [ block ] ⇐ A list of exprs.
        │    ┌┘
        │    │
       [ expr ] ── [ lit ] ⇐ A literal value; "base case" for the tree.
        │    │      ├── type
     ┌──┘    └──┐   └─ value
[ def ]        [ call ]
 │                   │
 ├─ [target] ← id    ├─ [target] ← id ⇐ An id is a pointer to another part of the tree.
 └── [value] ← expr  │                  It also contains the flags used in the definition.
                     └──── [arg] ← expr

Example Expr Tree
=================
[ block ]
 │
 ├─ [ def ]
 │   │
 │   ├─ [target] → hello
 │   └── [value] → [ lit ]
 │                  ├── type → str
 │                  └─ value → Hello, World
 ├─ [ call ]
 │   │
 │   ├─ [target] → print
 │   └──── [arg] → hello
 ...