04a readme and corrections

04a/Makefile

@@ -1,6 +1,8 @@
-all: out03
+all: out03 out04a README.html
 out03: in03 ../03/out02
 	../03/out02
+out04a: out03 in04a
+	./out03 in04a out04a
 %.html: %.md ../markdown
 	../markdown $<
 clean:

04a/README.md

@@ -0,0 +1,23 @@
+# stage 04a
+
+Rather than a compiler, this stage only consists of a simple [preprocessor](https://en.wikipedia.org/wiki/Preprocessor).
+In the future, we'll run our code through this program, then run its output
+through a compiler.
+
+It take lines like:
+```
+#define THREE d3
+```
+and then replaces `THREE` anywhere in the rest of the code with `d3`.
+I've provided `in04a` as a little example.
+Unlike previous programs, you can control the input and output file names
+without recompiling it. So to compile the example program:
+```
+make out03
+./out03 in04a out04a
+```
+
+Although it seems simple, this program will be very useful:
+it'll let us define constants and it'll work in any language.
+There really isn't much else to say about this program. With that,
+we can move on to [the next stage](../04b/README.md) which should be more exciting.

04a/in03

@@ -68,7 +68,7 @@ R=:line
 		; check if we reached the end of the line
 		C=1R
 		D=xa
-		?C=A:read_line
+		?C=D:read_line
 		; increment R, keep looping
 		R+=d1
 		!:process_line_loop

04a/in04a

@@ -1,7 +1,3 @@
-A+B=hello A adfhsakjfhjksah+B
-#define COLON d1
-#define SEMICOLON d2
-#define COMMA d3
-
-A=COLON
-1B=A
+#define H Hello,
+#define W world
+H W!