LFOPEN

(LFOPEN <filename> [<mode>])

Opens a file <filename> for reading/writing as specified with <mode>
(e.g., "r" for reading, "w" for writing etc). See standard C library 
function fopen(3) for details regarding the mode. If <mode> is not
given, the "r" mode is assumed. 

The function returns an S-expression derived from SExpressionStream.
More precisely, it creates SExpressionStreamFile object unless the 
mode parameter is not given; in this case it creates 
SExpressionStreamTextInput so that line counting information is available,
e.g., for the LispReader. It is recommended therefore not to specify the
mode parameter if you plan to use the LispReader functions, such as READ
and SAFEREAD.