HTDP 6.1 structure access

>From HTDP, section 6.1:

"To describe how posn-x, posn-y, and make-posn are related, we can use
equations that are roughly analogous to the equations that govern addition
and subtraction:

(posn-x (make-posn 7 0))
= 7

(posn-y (make-posn 7 0))
= 0"

http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-9.html#node_sec_6.1

I don't see the analogy. How is structure construction and access related to
the equations that govern addition and subtraction? Which equations?

The equations I can think of that govern addition and subtraction are
identity, inverse, commutativity and associativity:

a + 0 = a
a + (-a) = 0
a + b = b + a
(a + b) + c = a + (b + c)

Where is the rough analogy?

- Marlene


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