Add documentation for loops

docs/md/README_3_NON-DET.md

@@ -39,7 +39,7 @@ order. Click on a name to see how it is used.
 ## What can you do in Source §3 Non-Det?
 
 You can use all features of
-<a href="../source_3/">Source §3</a>, with the exception of loops, and all
+<a href="../source_3/">Source §3</a> and all
 features that are introduced in
 <a href="https://sicp.comp.nus.edu.sg/chapters/85">chapter 4.3</a> of the
 textbook.

docs/specs/source_3_nondet.tex

@@ -20,7 +20,7 @@ \section*{Changes}
 
 In the Source Academy implementation, the Source \S 3 Non-Det evaluator can only process one program at a time. If the evaluator is given a new program \textbf{before} all the outcomes of an existing program have been processed, then all remaining outcomes will be ignored. \newline
 
-Unlike Source \S 3, Source \S 3 Non-Det currently does not support loops.
+Unlike the implementations of Source \S 3,  the implementation of Source \S 3 Non-Det does not use a constant amount of memory for loops. Instead, memory usage increases linearly with the number of iterations. This is because each iteration can be nondeterministic. Each iteration's state must be kept in memory so that the language processor can backtrack to every iteration. Similarly, no tail call optimization is done, because every function application's state must be kept in memory to allow backtracking.
 
 
 \input source_bnf.tex
@@ -33,6 +33,8 @@ \section*{Changes}
 
 \input source_boolean_operators
 
+\input source_loops
+
 \input source_return_3
 
 \input source_names