Debugging functions

[Jim McDonald <jlm@lucid.com>]

   One of the main ways I use (dbg:arg ..) in Symbolics land is to change the
   value of an arg to get a program to continue.  For example, 
   	(setf (dbg:arg 3) 'foo).  
 
   Is this possible in Lucid land?

Yes.  Use the :S option, e.g.:

-> :S FOO 3

to set (well, actually rebind) the local named FOO to 3

You also might want to use :I FOO, which will inspect the indicated
local.

I just noticed that in LCL 3.0, the :S option is not mentioned when you 
type ?, but does appear for :??.  Some relevant lines:

-> :??
... many elided lines ...
:I k         Inspect local variable numbered k and set * to result.
:I k   var2  Inspect as above but also set var2 to result.
:I var       Inspect local variable named var and set * to result.
:I var var2  Inspect as above but also set var2 to result.
:L k         Print local variable numbered k and set * to its value.
:L k   var2  Print as above but also set var2 to the same value.
:L var       Print local variable named var and set * to its value.
:L var var2  Print as above but also set var2 to the same value.
:S k   exp   Evaluate exp and store result in local variable numbered k.
:S var exp   Evaluate exp and store result in local variable named var.
... more elided lines ...
-> 

The full list of options is strongly suggested reading if you do
more than occasional debugging.  It mentions several useful features:
trap on exit, unwinding the stack, pretty-printing, obtaining a
signalled condition, returning multiple values, etc.

  jlm