diff --git a/Makefile b/Makefile
index 46fd956..574911b 100644
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,7 @@ TOOLPREFIX := $(shell if riscv64-unknown-elf-objdump -i 2>&1 | grep 'elf64-big'
 	elif riscv64-linux-gnu-objdump -i 2>&1 | grep 'elf64-big' >/dev/null 2>&1; \
 	then echo 'riscv64-linux-gnu-'; \
 	else echo "***" 1>&2; \
-	echo "*** Error: Couldn't find an riscv64 version of GCC/binutils." 1>&2; \
+	echo "*** Error: Couldn't find a riscv64 version of GCC/binutils." 1>&2; \
 	echo "*** To turn off this error, run 'gmake TOOLPREFIX= ...'." 1>&2; \
 	echo "***" 1>&2; exit 1; fi)
 endif
diff --git a/user/grind.c b/user/grind.c
index 97a8a93..a01271e 100644
--- a/user/grind.c
+++ b/user/grind.c
@@ -48,13 +48,17 @@ rand(void)
 }
 
 void
-go()
+go(int which_child)
 {
   int fd = -1;
   static char buf[999];
   char *break0 = sbrk(0);
+  uint64 iters = 0;
   
   while(1){
+    iters++;
+    if((iters % 500) == 0)
+      write(1, which_child?"B":"A", 1);
     int what = rand() % 20;
     if(what == 1){
       close(open("a", O_CREATE|O_RDWR));
@@ -76,8 +80,12 @@ go()
       read(fd, buf, sizeof(buf));
     } else if(what == 9){
       mkdir("a");
+      close(open("a/a", O_CREATE|O_RDWR));
+      unlink("a/a");
     } else if(what == 10){
       mkdir("b");
+      close(open("b/b", O_CREATE|O_RDWR));
+      unlink("b/b");
     } else if(what == 11){
       unlink("b");
       link("a", "b");
@@ -88,6 +96,9 @@ go()
       int pid = fork();
       if(pid == 0){
         exit(0);
+      } else if(pid < 0){
+        printf("grind: fork failed\n");
+        exit(1);
       }
       wait(0);
     } else if(what == 14){
@@ -96,6 +107,9 @@ go()
         fork();
         fork();
         exit(0);
+      } else if(pid < 0){
+        printf("grind: fork failed\n");
+        exit(1);
       }
       wait(0);
     } else if(what == 15){
@@ -108,6 +122,9 @@ go()
       if(pid == 0){
         close(open("a", O_CREATE|O_RDWR));
         exit(0);
+      } else if(pid < 0){
+        printf("grind: fork failed\n");
+        exit(1);
       }
       kill(pid);
       wait(0);
@@ -116,8 +133,34 @@ go()
       if(pid == 0){
         kill(getpid());
         exit(0);
+      } else if(pid < 0){
+        printf("grind: fork failed\n");
+        exit(1);
       }
       wait(0);
+    } else if(what == 19){
+      int fds[2];
+      if(pipe(fds) < 0){
+        printf("grind: pipe failed\n");
+        exit(1);
+      }
+      int pid = fork();
+      if(pid == 0){
+        fork();
+        fork();
+        if(write(fds[1], "x", 1) != 1)
+          printf("grind: pipe write failed\n");
+        char c;
+        if(read(fds[0], &c, 1) != 1)
+          printf("grind: pipe read failed\n");
+        exit(0);
+      } else if(pid < 0){
+        printf("grind: fork failed\n");
+        exit(1);
+      }
+      close(fds[0]);
+      close(fds[1]);
+      wait(0);
     }
   }
 }
@@ -125,6 +168,9 @@ go()
 int
 main()
 {
+  unlink("a");
+  unlink("b");
+  
   int pid1 = fork();
   if(pid1 < 0){
     printf("grind: fork failed\n");
@@ -132,7 +178,7 @@ main()
   }
   if(pid1 == 0){
     rand_next = 31;
-    go();
+    go(0);
     exit(0);
   }
 
@@ -143,7 +189,7 @@ main()
   }
   if(pid2 == 0){
     rand_next = 7177;
-    go();
+    go(1);
     exit(0);
   }