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Some of these operations fail depending on the availability debugging
information. In the most severe case, when someone saved a Lisp image
stripping all debugging data structures, no operations are valid. In
this case, even backtracing and finding frames is impossible. Some
interfaces can simply return values indicating the lack of information,
or their return values are naturally meaningful in light missing data.
Other routines, as documented below, will signal
serious-conditions when they discover awkward situations. This
interface does not provide for programs to detect these situations other
than by calling a routine that detects them and signals a condition.
These are serious-conditions because the program using the interface
must handle them before it can correctly continue execution. These
debugging conditions are not errors since it is no fault of the
programmers that the conditions occur.