R4RS 6.10.2  (read-char)             ==>  char | eof-object
             (read-char input-port)  ==>  char | eof-object

Returns the next character available from the input port, updating
the port to point to the following character. If no more characters
are available, an end of file object is returned. INPUT-PORT may
be omitted, in which case it defaults to the value returned by
CURRENT-INPUT-PORT.
