The way that C expresses it elegantly is via the comparison operator. The author is concerned that people try to be clever and use subtraction instead. Here is how the author corrected this error by using comparison operators instead of subtracting (in the tree man page):

    int
    intcmp(struct node *e1, struct node *e2)
    {
  -	  return (e1->i - e2->i);
  +	  return (e1->i < e2->i ? -1 : e1->i > e2->i);
    }

The ternary operator is necessary, because in C a relational operator is evaluated to 0 (false) or 1 (true). So, the author added a special case for returning a negative value (indicating the first argument should be sorted before the second).