until &fail do write("SPAM")   # always fails, needs succeed to break
...
while write("SPAM")            # always succeeds, needs failure to break
...
every write(|"SPAM")           # generator always succeeds, needs failure to break
...
while write(|"SPAM")           # this is a common mistake that results in an endless loop
...
while write(1 to 5)            # a clearer version of the same mistake that generates endless 1's