Anyway, all programming languages have some form of procedure. If no
one has thought it important enough to add such a feature to their
debugging tools,
It is not exactly true I think. For example in the GNU C debugger it is definitely possible to call
any procedure (with great difficulties obviously because of the parameters and the global variables).
For some languages such as Haskell it is intrinsically possible to test any function in isolation.
For languages such as Forth the system of stacks make it possible to
call any function in isolation easily I think because of the stack system
and eventually embedded languages like lua make it possible to call procedure in isolation as well.
--
FL