Removing the global function definition from a traced symbol (function) makes
UNTRACE with no arguments (i.e., the intent being of untracing everything
that's being traced) to signal an UNDEFINED-FUNCTION error.
As it is now (SBCL 18.104.22.168), it is impossible to remove the funbound symbol
from the trace list.
If the same function is redefined and traced again, the same symbol is
inserted into the list of traced functions; calling UNTRACE with no arguments
now will warn about function being not traced. (What is a function that is
not being traced doing in the trace list, anyway?)
A sketch of the test case is attached.