CM error messages

When the CM can't find a component, it gives very cryptic messages. I 
didn't realize that in 110.60, the tdp-util library isn't installed by 
default, so when I tried to use the BackTrace library, I got the 
following error message:

% ml-build -Ctdp.instrument=true \$smlnj-tdp/back-trace.cm  main.cm 
Main.main main.heap
Standard ML of New Jersey v110.60 [built: Mon Nov 20 09:30:59 2006]
[scanning $smlnj-tdp/back-trace.cm]
/Applications/smlnj/bin/sml: Fatal error -- Uncaught exception Io  with 
<unknown> raised at Basis/Implementation/IO/text-io-fn.sml: 784.25-784.71

It took me a long time to uncover what was going wrong: the CM didn't 
recognize the "smlnj-tdp" variable because that component wasn't 
installed, and consequently replaced it with something like 
$Undef<smlnj-tdp>. Then the system proceeded to try to find 
$Undef<smlnj-tdp>/back-trace.cm and failed to find it, whereupon it 
raise an Io exception. (Did I get that right?)

It would be very useful if the CM would raise an exception indicating 
that it doesn't recognize the variable, with an informative error 
message indicating that a particular component needs to be installed.

Thanks,
Dave

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