The zsh-lovers man page contains the following example.

# Show me all the .c files for which there doesn't exist a .o file.
$ c=(*.c) o=(*.o(N)) eval 'ls ${${c:#(${~${(j:|:)${o:r}}}).c}:?done}'

What's with the ugly dollar-sign prompt? I'd do it this way instead.

print *.c(e_' ! -e $REPLY:r.o '_)