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

Returns the next character available from the input port, without
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.

Note: The value returned by a call to PEEK-CHAR is the same as the
value that would have been returned by a call to READ-CHAR with the
same port. The only difference is that the very next call to READ-CHAR
or PEEK-CHAR on that port will return the value returned by the
preceding call to PEEK-CHAR. In particular, a call to PEEK-CHAR on
an interactive port will hang waiting for input whenever a call to
READ-CHAR would have hung.
