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docs: add section about units to the pascal programming guide

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@ -235,6 +235,85 @@ In Wirth Pascal, labels must be numbers. Other Pascal dialects also allow normal
Tridora-Pascal only allows identifiers as labels.
## Units
Units are the method to create libraries in Tridora-Pascal, that is, codes module that can
be reused in other programs.
Tridora-Pascal follows the unit syntax that has been established in UCSD-Pascal and is also
used in Turbo Pascal.
Units are imported with the *USES* keyword, right after the *PROGRAM* statement.
Multiple units can be imported by separating the unit names with commas.
There are some differences: In Tridora-Pascal, the unit file does not contain the interface
section, only the implementation section. The interface section is instead placed into a
separate file with the extension *.inc*, without any *UNIT* or *INTERFACE* keywords.
This file will be included by the compiler and should contain
procedure or function declarations (as *EXTERNAL*). It can also contain *TYPE*,
*CONST* and *VAR* statements.
All Pascal symbols of the unit are imported into the main program. There
is no separate namespace for units.
### Using an Existing Unit
An existing unit is imported with the *USES* statement that must be placed
immediately after the *PROGRAM* statement.
The compiler will look for an include file with the unit name and an *.inc* extension.
It will also
tell the assembler to include an assembly language file for each
unit. The filename must be the unit name plus an *.s* extension.
Since there is no linker in Tridora-Pascal, all imported units will be
assembled together with the main program.
The compiler looks for unit *.inc* and *.s* files in the current volume or
in the *SYSTEM* volume.
### Compiling a Unit
A unit implementation file should start with a *UNIT* statement instead of a *PROGRAM*
statement.
It should be compiled, not assembled.
When building a program that uses units, the assembler will include an assembly language
file for each unit.
It is possible to write units in assembly language. This is done by
directly providing the *.s* file and creating an *.inc* file with
the *EXTERNAL* declarations matching the assembly language
file.
#### Example
```pascal
(* UnitExamples.pas *)
program UnitExample;
uses hello;
begin
sayHello('unit');
end.
```
#### Example Unit Implementation File
```pascal
(* hello.pas *)
unit hello;
implementation
procedure sayHello(s:string);
begin
writeln('hello, ', s);
end;
end.
```
#### Example Unit Include File
```pascal
(* hello.inc *)
procedure sayHello(s:string); external;
```
## Compiler Directives
Tridora-Pascal understands a small number of compiler directives which are introduced as usual with a comment and a dollar-sign. Both comment styles can be used.