Previous behavior of Mathomatic:

1-> y=x

#1: y = x

1-> derivative z ; Fail!
Variable not found; the derivative would be zero.
Command usage: derivative ["nosimplify"] [variable or "all"] [order]
1-> y=2

#2: y = 2

2-> derivative x ; Fail!
Current expression contains no variables; the derivative would be zero.
Command usage: derivative ["nosimplify"] [variable or "all"] [order]
2->

Differentiation failed with a helpful error message and useless command usage info when the result is zero. That was wrong.

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Current behavior of Mathomatic, available in the development version and the next release:

1-> y=x

#1: y = x

1-> derivative z ; Success!
Warning: Variable not found; the derivative will be zero.
Differentiating the RHS with respect to (z) and simplifying...

#2: y' = 0

2-> y=2

#3: y = 2

3-> derivative x ; Success!
Warning: Current expression contains no variables; the derivative will be zero.
Warning: Variable not found; the derivative will be zero.
Differentiating the RHS with respect to (x) and simplifying...

#4: y' = 0

4->

Mathomatic succeeds now when the derivative is zero, giving a warning message (or two).

Displaying command usage info whenever a command failed will not happen anymore. Command usage info is displayed by the help command and when something has been entered incorrectly on the command line.

More progress, I think. There have been many important improvements and fixes to Mathomatic this year. I am pleased, and thankful for the support I get. :-)