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Construct an addition of any number of expressions.
If there are no expressions, we return the constant expression 0.
If there is just one expression, we return it.
If there are two or more expressions, we return their left-associated sum.
Macro:
(defmacro pf+ (&rest exprs) (cond ((endp exprs) '(pfconst 0)) ((endp (cdr exprs)) (car exprs)) (t (yyyjoin 'expression-add (rev exprs)))))