While reading output, more than 2000 bytes can force
             earlier  bytes  to  be  "forgotten".   This  may  be
             changed with the  function  match_max.   (Note  that
             excessively  large  values can slow down the pattern
             matcher.)  If patlist  is  full_buffer,  the  corre-
             sponding  body  is  executed if match_max bytes have
             been received and no other  patterns  have  matched.
             Whether  or not the full_buffer keyword is used, the
             forgotten     characters     are     written      to
             expect_out(buffer).

             taken from 'man expect'