Update 2012/tromp

Format and typo fix README.md.

Un-rot13 the how13 and make it how.md. The reversal of rot13 was
discussed on GitHub at Landon's request.

One link was fixed.

Added other links. I was sorely tempted to make two links for garbage
collector, garbage == Windows and collector == the actual garbage
collector :-) but I decided that that's not my place so I didn't do it
as much as I wanted to (and I dearly wanted to!).

In some places there probably should be links again but since they were
linked to earlier in the document I'm going to pass on it for now.

I believe but am not certain that with this commit with the exception of
2012/README.md that 2012 is finished at least with format fixes in
README.md files (unless I come across something when trying out the
entries later on).

2012/tromp/how.md

@@ -0,0 +1,66 @@
+# Spoiler
+
+The program implements a so called [Krivine
+machine](https://en.wikipedia.org/wiki/Krivine_machine), enhanced with a [basic
+input/output system](https://en.wikipedia.org/wiki/BIOS) and a reference
+counting [garbage collector](https://en.wikipedia.org/wiki/Waste_collector). The
+Krivine machine is a stepwise transformer of a global state consisting of the
+current term, the current environment (a list of closures), and a stack of
+continuations (see the 3rd reference for details).
+
+The term space holds [lambda
+calculus](https://en.wikipedia.org/wiki/Lambda_calculus) terms in the following
+format. 0 denotes some form of IO; the only four occurrences are at:
+
+```
+  C=13: recognizes end of a byte
+  C=20: recognize output of 0 bit
+  C=21: recognize output of 1 bit
+  C=end: input list, expanded lazily
+```
+
+`1` denotes a variable, with next entry its de [Bruijn
+index](https://en.wikipedia.org/wiki/De_Bruijn_index).
+
+`2` denotes application with next entry the size of the term being applied.
+
+`3` denotes lambda abstraction.
+
+
+A closure consists of the following items:
+
+0. pointer to the next closure (as part of an environment), or next record on
+free list.
+1. reference count
+2. pointer to environment
+3. lambda term (index in term space)
+
+Here's what each variable do:
+
+```
+L	Lambda term space
+m	mode: 0 for bits and 7 for bytes
+b	bits remaining unread in I, -1 on EOF
+D	Desirable temporary
+c	continuation context
+a	active environment pointer
+C	Current term index
+U	Ultimate term pointer
+u	utility entry
+B	Born to be free-list of closures
+I	Input character
+O	Output character
+e	entry of current term
+```
+
+Here's what each of the 6 auxiliary functions do:
+
+```
+s	variable lookup in active environment with de Bruijn index u
+S	gets one bit of input, setting b to -1 on EOF or to remaining number of bits in current byte
+k	copy fragment from string literal holding lambda terms xor 46 into current end of term space
+x	expand input, adding 11 (resp. 99) entries to term space for bit (resp. byte) mode
+p	parses BLC-encoded lambda term using S(), stores results in term space and returns length
+d	decrease reference counter, add record to free list on reaching zero
+```
+