cameo: completed support for dril/mill conversions

- README: changed the term "drill/mill substitution" to "drill/mill
  conversion"
- README: described the drill/mill conversion commands
- lang.y: "drill" and "mill" worked on the wrong (empty) list
- ops.c (half_circle): fixed transcription error

cameo/README

@@ -138,9 +138,41 @@ changes this to cutting a "dogbone" hole such that material is also
 removed up to the corner point.
 
 
-Drill/mill substitution:
+Drill/mill conversion:
 
   drill <min-diameter> <max-diameter>
   mill <diameter> <step>
   remainder
   empty
+
+"drill" and "mill" search for paths that can be processed with the
+specified tool and generate the corresponding toolpaths. After "drill"
+or "mill", only the toolpaths for this tool remain. The paths that
+have been converted are removed and the paths for which the tool is
+unsuitable are moved to a separate list, the remainder list.
+
+"remainder" clears the list of paths and sets the list of paths to
+the remainder list. Note that "clear" does not affect the remainder
+list.
+
+"empty" does nothing if the list of paths is empty and makes cameo
+exit with an error if there are paths in the list. This can be used
+to ensure that all paths in a file have been converted.
+
+Example:
+
+  excellon example.drl
+  drill 7mil 13mil
+  write drill.gp
+  remainder
+  mill 0.8mm 0.01mm
+  write mill.gp
+  remainder
+  empty
+
+In this example, we first load a drill file. Then we generate drill
+commands for all simple holes with a diameter between 7 mil and 13 mil
+and write the toolpaths to the file "drill.gp". Next, we retrieve the
+remaining paths, generate toolpaths for a 0.8 mm endmill, and write
+them to the file "mill.gp". Finally, we check that all paths have been
+processed.

cameo/lang.y

@@ -247,7 +247,7 @@ command:
 
 			remain = paths;
 			paths = NULL;
-			walk = &paths;
+			walk = &remain;
 			while (*walk)
 				walk =
 				    classify(walk, try_drill(*walk, $2, $3));
@@ -258,7 +258,7 @@ command:
 
 			remain = paths;
 			paths = NULL;
-			walk = &paths;
+			walk = &remain;
 			while (*walk)
 				walk = classify(walk, try_mill(*walk, $2, $3));
 		}

cameo/shape.c

@@ -39,7 +39,7 @@ static void half_circle(double cx, double cy, double rx, double ry, double z,
 	m[3] = m[0];
 
 	for (a = 0; a < M_PI; a += s) {
-		path_add(path, cx+x, cy+x, z);
+		path_add(path, cx+x, cy+y, z);
 		tmp = x*m[0]+y*m[1];
 		y = x*m[2]+y*m[3];
 		x = tmp;