The fossil test-delta-apply command requires three arguments, but --help does not document this:

% fossil test-delta-apply --help
Usage: fossil test-delta-apply FILE1 DELTA

Apply DELTA to FILE1 and output the result.

If run like this, you get an error:

fossil test-delta-apply before delta
Usage: fossil test-delta-apply ORIGIN DELTA TARGET

The patch below updates the message in the spirit of fossil test-delta-create --help:

% ./fossil test-delta-apply --help
Usage: fossil test-delta-apply FILE1 DELTA FILE2

Apply DELTA to FILE1.  Store the result in FILE2.

% fossil diff --from trunk
Index: src/deltacmd.c
==================================================================
--- src/deltacmd.c
+++ src/deltacmd.c
@@ -143,13 +143,13 @@
 }

 /*
 ** COMMAND: test-delta-apply
 **
-** Usage: %fossil test-delta-apply FILE1 DELTA
+** Usage: %fossil test-delta-apply FILE1 DELTA FILE2
 **
-** Apply DELTA to FILE1 and output the result.
+** Apply DELTA to FILE1.  Store the result in FILE2.
 */
 void delta_apply_cmd(void){
   Blob orig, target, delta;
   if( g.argc!=5 ){
     usage("ORIGIN DELTA TARGET");