todo

__Done:__
+ Variables
  + Numbers (INTEGER & REAL)
  + STRING
  + BOOLEAN
  + Constants
  + Arrays
    + Multi-dimensional arrays
+ Operators
  + Plus
  + Minus
  + Times
  + Equals
  + Assign
  + Modulus
+ PROCEDUREs
  + Parameters BYVAL
  + CALL
+ Loops
  + FOR loops
+ If statements
  + IF
  + Else
+ Comments
+ I/O
  + INPUT
  + OUTPUT
  + Files
    + OPEN/CLOSE
    + Read
    + Write, Append
+ FUNCTIONs
  + RETURN

__TBC:__
- CASE statements
  - Ranges
  - Must contain OTHERWISE
- Parameters BYREF
- Variable scopes
- REPEAT...UNTIL
- Types
  - Custom types
    - Pointer types?
    - Composite types (structs)
    - Uncomposite (enums)
  - Validating built-in types
- I/O
  - Files
    - Binary (random) files 
- OOP?