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(defun conv (exp)
  (cond 
   ((not (listp exp)) "x")
   ((equal (first exp) '+)
    (concatenate 'string "Plus[" (conv (second exp)) "," (conv (third exp)) "]"))

The lines above are from the file 'convertToMathematic.lisp', lying in the 
ec/app/regression/ directory.

The third line, '((not (listp exp)) "x")',seems to mean that, if what I get is not 
a list expression, then I will replace it with "x". This file works fine with a 
function who has only one variable. However, if  the intention is to transform 
a Lisp s-expression with more than one variables, like "x","y" and "z", then 
what should I do to make the change?