Each element has the format (PATTERN . VAL), where PATTERN determines which network sockets the element applies to. PATTERN should be either a port number or a regular expression matching some network service names.
VAL is a coding system, a cons cell containing two coding systems, or a function symbol. If VAL is a coding system, it is used for both decoding what received from the network stream and encoding what sent to the network stream. If VAL is a cons cell containing two coding systems, the car specifies the coding system for decoding, and the cdr specifies the coding system for encoding.
If VAL is a function symbol, the function must return a coding system or a cons cell containing two coding systems, which is used as above.