added the Assignments chapter

Assignments.md

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+# Assignments
+
+Assignments are programming tasks that can be tested by a worker after a user 
+submits their solution.
+
+## Configuration format
+
+An assignment is described by a YAML file that contains information on how to 
+build, run and test it. The testing process is divided into stages.
+
+### Tests
+
+
+### Stages
+
+A stage is a logical unit of the testing process. It specifies how to do a step 
+in the build process and how to test if it behaves correctly. The following is 
+contained in its configuration:
+
+- **Name** - a unique string identifier of the stage
+- **Build command** (optional) - used to prepare the submitted files for this 
+  test stage
+- **Test list** - specifies the tests to be run during this stage
+- **Test command** - used to run one specific test
+- **Error policy** (optional) - what should we do when a test fails?
+	- **interrupt** the stage (default)
+	- **continue** with the next test
+	- **jump** to another stage (TODO cycle detection?)
+- **Success policy** (optional) - what to do when all tests pass?
+	- **jump** to another stage (the next one by default)
+	- **end** the evaluation, even if there are still unprocessed stages
+
+## Case study
+
+We present some of the courses that might use ReCodEx to evaluate homework 
+assignments and outline the setup of the evaluation with respect to the concept 
+of stages.
+
+### Simple programming exercises
+
+For example introductory programming courses such as Programming I or 
+Programming in Java.
+
+In the simplest case we only need one stage that builds the program and passes 
+the test inputs to its standard input. We will use the C language for this 
+example. The build command is `gcc source.c`, the test command is `./a.out`.
+
+### Compiler principles
+
+This course uses multiple tools in a pipeline-like fashion - for example `flex` 
+and `bison`.
+
+We create a stage for each of the steps of this pipeline - we run flex and test 
+the output, then we run bison and do the same.
+
+### XML technologies
+
+In this course, students choose a topic they model using XML - for example a 
+library or a bulletin board. During the semester, they expand this project by 
+adding XSLT transformations, XQuery scripts, XPath queries, etc. These are 
+tested against fixed requirements (e.g. using some particular language 
+constructs).
+
+This course already has a rather sophisticated application for testing homework 
+assignments, so we only include it for demonstration purposes.
+
+Because every assignment focuses on a different technology, we would need a new 
+type of stage for each one. These stages would only run some checker programs 
+against the submitted sources (and possibly try to check their syntax etc.).
+
+### Non-procedural programming
+
+This course is different from other programming courses, because it only teaches 
+input/output manipulation by the end of the semester. In their assignments, 
+students are mostly required to write a function/predicate that behaves 
+according to a specification (e.g. appends an item at the end of a list).
+
+Due to this, we need to take the function submitted by a student and combine it 
+with a snippet of code that reads the standard input and calls the submitted 
+function. This could be achieved by setting the build command.
+
+### Operating systems
+
+The operating systems course requires students to work on a simple OS kernel 
+that is then run in a MIPS simulator called `msim`. There are various tests that 
+check if the student's implementation of core OS mechanisms is correct. These 
+tests are compiled into the kernel.
+
+Each of these tests could be represented by a stage that compiles the kernel 
+with the test and then runs it against different configurations of `msim`.