4. The motor-vehicle lock system defined in claim 1 wherein the transmitter means includes a code register capable of outputting a predetermined sequence of code words and steppable through the sequence to output the code words sequentially one at a time, a clock connected to the register for periodically stepping it through the code-word sequence, and output means for modulating a signal with the code word currently being outputted by the register and for transmitting the modulated signal as the main signal; the receiver means including a code register like the transmitter code register, capable of outputting the predetermined sequence of code words, and steppable through the sequence to output the code words sequentially one at a time, a clock like the transmitter clock and connected to the receiver code register for periodically stepping it through the code-word sequence substantially synchronously with the transmitter clock and transmitter register, and input means connected to the receiver code register and to the controller for receiving and demodulating a transmitted main signal, extracting a main code word from the demodulated signal, comparing the extracted main code word with the code word currently being generated by the receiver code register, and generating a release signal and sending it to the controller when the extracted code word not only corresponds to a code word in the code-word sequence but also lies no more than a predetermined number of code words in the sequence away from the code word currently being outputted by the receiver code register.
4. The motor-vehicle lock system defined in claim 1 wherein the transmitter means includes a code register capable of outputting a predetermined sequence of code words and steppable through the sequence to output the code words sequentially one at a time, a clock connected to the register for periodically stepping it through the code-word sequence, and output means for modulating a signal with the code word currently being outputted by the register and for transmitting the modulated signal as the main signal; the receiver means including a code register like the transmitter code register, capable of outputting the predetermined sequence of code words, and steppable through the sequence to output the code words sequentially one at a time, a clock like the transmitter clock and connected to the receiver code register for periodically stepping it through the code-word sequence substantially synchronously with the transmitter clock and transmitter register, and input means connected to the receiver code register and to the controller for receiving and demodulating a transmitted main signal, extracting a main code word from the demodulated signal, comparing the extracted main code word with the code word currently being generated by the receiver code register, and generating a release signal and sending it to the controller when the extracted code word not only corresponds to a code word in the code-word sequence but also lies no more than a predetermined number of code words in the sequence away from the code word currently being outputted by the receiver code register. 5. The motor-vehicle lock system defined in claim 4, further comprising a random-number generator having an output and energizeable to produce a random number corresponding to one of the code words at its output; and initializing means for at least temporarily connecting the output of the random-number generator to both of the registers for resetting same to output the code word corresponding to the random number of the generator.