Added big-picture info to the design document

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 #+TITLE: 7Units Design Document
 #+SUBTITLE: For version 0.4.0-alpha.1
-#+DATE: 2022 May 18
+#+DATE: 2022 June 18
 #+LaTeX_HEADER: \usepackage[a4paper, lmargin=25mm, rmargin=25mm, tmargin=25mm, bmargin=25mm]{geometry}
 #+LaTeX_HEADER: \usepackage{xurl}
 #+LaTeX: \newpage
 
 * Introduction
   7Units is a program that can convert between units.  This document details the internal design of 7Units, intended to be used by current and future developers.
+* System Overview
+** Packages of 7Units
+   7Units splits its code into three main packages:
+   - ~sevenUnits.unit~ :: The [[*Unit System Design][unit system]]
+   - ~sevenUnits.utils~ :: Extra classes that aid the unit system.
+   - ~sevenUnitsGUI~ :: The [[*Front-End Design][front end]] code
+   ~sevenUnits.unit~ depends on ~sevenUnits.utils~, while ~sevenUnitsGUI~ depends on both ~sevenUnits~ packages.  There is only one class that isn't in any of these packages, ~sevenUnits.VersionInfo~.
+** Major Classes of 7Units
+   - [[*Unit Classes][sevenUnits.unit.Unit]] :: The class representing a unit
+   - [[*The Unit Database][sevenUnits.unit.UnitDatabase]] :: A class that stores collections of units, prefixes and dimensions.
+   - [[*The View][sevenUnitsGUI.View]] :: The class that handles interaction between the user and the program.
+   - [[*The Presenter][sevenUnitsGUI.Presenter]] :: The class that handles communication between the ~View~ and the unit system.
+** Process of Unit Conversion
+   1. The ~View~ triggers a unit conversion method from the ~Presenter~.
+   2. The ~Presenter~ gets raw input data from the ~View~'s public methods (from unit, to unit, from value).
+   3. The ~Presenter~ uses the ~UnitDatabase~'s methods to convert the raw data from the ~View~ into actual units.
+   4. The ~Presenter~ performs the unit conversion using the provided unit objects.
+   5. The ~Presenter~ calls the ~View~'s methods to show the result of the conversion.
+** Process of Expression Conversion
+   1. The ~View~ triggers a unit conversion method from the ~Presenter~.
+   2. The ~Presenter~ gets raw input data from the ~View~'s public methods (unit conversion: from unit, to unit, from value; expression conversion: input expression, output expression).
+   3. The ~Presenter~ uses the ~UnitDatabase~'s methods to parse the expressions, converting the input into a ~LinearUnitValue~ and the output into a ~Unit~.
+   4. The ~Presenter~ converts the provided value into the provided unit.
+   5. The ~Presenter~ calls the ~View~'s methods to show the result of the conversion.
 * Unit System Design
   Any code related to the backend unit system is stored in the ~sevenUnits.unit~ package.