Improve the posix preprocessor.

This fixes token pasting, making it expand when
it should expand, and paste before expansion when
it should paste before expanding.

	#define CAT(a, b) a ## b
	#define BAR	3
	#define FOO	CAT(BAR, 3)
	FOO

now produces 33, while

	#define CAT(a, b) a ## b
	#define EOF	(-1)
	#define NOP(x)	x
	NOP(CAT(foo, EOF))
	CAT(,EOF)
	CAT(,)

produces

	fooEOF
	(-1)
	<empty>

respectively.

1 files changed, 61 insertions, 4 deletions

diff --git a/sys/src/cmd/cpp/test.c b/sys/src/cmd/cpp/test.c
index 3fbd6bdd6..451e364f4 100644
--- a/sys/src/cmd/cpp/test.c
+++ b/sys/src/cmd/cpp/test.c
@@ -1,4 +1,61 @@
-#define M1()
-#define M2(A1) A1()
-M2(M1)
-M2(P1)
+#define NOP(x) x
+#define CAT(a, b) a ## b
+#define EOF	(-1)
+x NOP(CAT(foo, EOF)) y
+x NOP(CAT(EOF, foo)) y
+x CAT(, EOF) y
+y CAT(foo,) x
+x CAT(,foo) y
+X NOP(CAT(,)) y
+
+#define NCAT(a)	foo ## a
+NCAT(bar)
+
+#define XCAT(a)	## a
+foo XCAT(bar)
+
+#define CAT3(foo)	a##foo##b
+CAT3(blah)
+
+#define BAR	3
+#define FOO	CAT(BAR, 3)
+FOO
+
+/*
+ * CURRENTLY BROKEN:
+ *     __VA_ARGS__ requires at least one item.
+ *     It should accept an empty list.
+#define xprint(a, ...)	print(a, __VA_ARGS__)
+xprint("hi", "there")
+xprint("hi")
+*/
+
+#define C	a,b
+#define X(a)	a
+#define Y	X(C)
+Y
+
+#define    x          3
+#define    f(a)       f(x * (a))
+#undef     x
+#define    x          2
+#define    g          f
+#define    z          z[0]
+#define    h          g(~
+#define    m(a)       a(w)
+#define    w          0,1
+#define    t(a)       a
+#define    p()        int
+#define    q(x)       x
+#define    r(x,y)     x ## y
+#define    str(x)     # x
+f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
+g(x+(3,4)-w) | h 5) & m
+(f)^m(m);
+/*
+ * CURRENTLY BROKEN:
+ *     mac() needs at least one argument.
+ *     It should treat no args as a single empty arg list.
+p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
+char c[2][6] = { str(hello), str() };
+*/
\ No newline at end of file