Programming in the Debugger

Here's an anecdote I heard once about Minsky.  He was showing a
student how to use ITS to write a program.  ITS was an unusual
operating system in that the `shell' was the DDT debugger.  You ran
programs by loading them into memory and jumping to the entry point.
But you can also just start writing assembly code directly into memory
from the DDT prompt.  Minsky started with the null program.
Obviously, it needs an entry point, so he defined a label for that.
He then told the debugger to jump to that label.  This immediately
raised an error of there being no code at the jump target.  So he
wrote a few lines of code and restarted the jump instruction.  This
time it succeeded and the first few instructions were executed.  When
the debugger again halted, he looked at the register contents and
wrote a few more lines.  Again proceeding from where he left off he
watched the program run the few more instructions.  He developed the
entire program by 'debugging' the null program.