Adds initial draft for separate compilation

sources/modules/compilation-model/separate-compilation.md

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+## C++ compilation model: Separate Compilation {#separate-comp}
+
+_Skeleton descriptions are typeset in italic text,_
+_so please don't remove these descriptions when editing the topic._
+
+### Overview
+
+_Provides a short natural language abstract of the module’s contents._
+_Specifies the different levels of teaching._
+
+This module outlines the issues involved in using separate compilation on multiple program files.
+
+------------------------------------------------------------------------
+Level             Objective
+----------------- ------------------------------------------------------
+Foundational:      Awareness of the existence of separate compilation
+
+Main:              Awareness of supporting mechanisms and tools
+
+Advanced:          Awareness of technicalities and tools
+
+------------------------------------------------------------------------
+
+### Motivation
+
+_Why is this important?_
+_Why do we want to learn/teach this topic?_
+
+Any non-trivial program will span more than one source file. This makes separate compilation necessary. Separate compilation also leads to quick build times in projects under development.
+
+### Topic introduction
+
+_Very brief introduction to the topic._
+
+### Foundational: Using
+
+#### Background/Required Knowledge
+
+A student: 
+
+1. needs to have the text editor skills to split a single-file program into multiple files.
+2. needs to be able to split function and classes into their declaration and definition. [[C++ object model: declarations]][1] [[C++ object model: Definitions]][2]
+
+
+#### Student outcomes
+
+_A list of things "a student should be able to" after the curriculum._
+_The next word should be an action word and testable in an exam._
+_Max 5 items._
+
+A student should be able to:
+
+1. split a single file program into main, one or more auxiliaries, and header files (or modules), all in one directory
+2. compile and link a program that is spread over multiple files (again, all in one directory), either on the commandline or with a build system.
+3. describe the functions of the various files involved: source, object, executable.
+
+
+#### Points to cover
+
+* Necessity of having function be declared at the locus of use
+* Solving this by explicit declaration or by using header files.
+
+#### Caveats
+
+_This section mentions subtle points to understand, like anything resulting in
+implementation-defined, unspecified, or undefined behavior._
+
+1. Explain differences (including in desirability) between having explicit declarations of functions in the main program versus using header files or modules.
+2. Compilation on a single commandline versus multiple. Resolution order when linking.
+3. What needs to be recompiled if only a function is altered? Altered in use syntax versus altered only in semantics.
+
+### Main: implementing
+
+#### Background/Required Knowledge
+
+* All of the above.
+
+#### Student outcomes
+
+A student should be able to:
+
+1. Use compile flags, including `-I` and `-L` to specify search paths.
+2. Make header files for their own code, including using header guards.
+3. Use include directives for the headers of external libraries.
+4. Be able to declare `extern` variables.
+
+#### Caveats
+
+1. Build systems make things both easier and harder.
+
+#### Points to cover
+
+* Cover the mechanisms for include and library paths to your own code, as well as discovery mechanisms for external libraries.
+
+### Advanced
+
+_These are important topics that are not expected to be covered but provide
+guidance where one can continue to investigate this topic in more depth._
+
+1. Language inter-operability: the `extern "C"` mechanism.
+2. Linker conventions including name mangling.
+3. The `nm` tool for inspection object files and libraries, including de-mangling.
+4. Understand the issues involved in deciding between include guards and `#pragma once`.
+
+[1]: ../object-model/declarations.md
+[2]: ../object-model/definitions.md