sol-1.4.tex

In the given procedure, the compound expression used as operator is
\begtt\scm
(if (> b 0) + -)
\endtt
This expression simply evaluates to the operator  `+` or the operator `-`, depending on the value of \,`b`, whether it is positive or not.  Therefore, the procedure is equivalent to
\begtt\scm
(define (a-plus-abs-b a b)
   (if (> b 0) 
       (+ a b)
       (- a b)))
\endtt
and also to
\begtt\scm
(define (a-plus-abs-b a b)
   (+ a (if (> b 0)
            b
            (- b))))
\endtt
hence the name `a-plus-abs-b`.