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Specification of Reactive Hardware/Software Systems - Electronic ...

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254 Data Part <strong>of</strong> POOSL<br />

For readers with a more mathematical background it may be interesting to observe that<br />

that ¡<br />

continuous function on ¡<br />

))¢¤£ . By elementary fixed point theory it then follows<br />

(Dom(¡<br />

§ that FIX(¦ ¡¤£¥ = ), FIX(¦ ¡¤£¥ where ) denotes the least fixed point ¦ ¡¤£¥ <strong>of</strong> .<br />

(Dom(¡ ))¢¤£ is a complete lattice with least element ¥ , and that each ¦ ¡¤£¥ is a<br />

We are now able to formulate a definition <strong>of</strong> function copy. Let <br />

Sys-structure. Then<br />

where<br />

copy( <br />

¤ ¥ ¡ ¥<br />

£<br />

<br />

<br />

¤ ¡ ¡ ¤ ¡ ¡<br />

¥ ¤ ¡ <br />

¥ )<br />

¡¤£¥ (¤ n ¡ ) § n¦ 0<br />

¤ ¥ ¡ ¥<br />

¡<br />

¡ StrucSys be a<br />

¨ ¡<br />

The elements <strong>of</strong> the range <strong>of</strong> function copy are characterised by a number <strong>of</strong> interesting<br />

properties:<br />

Proposition 1<br />

Let <br />

¥<br />

¡<br />

¥ ¡ StrucSys and let copy( ¤ ¡ <br />

¡ ¡<br />

¥ ) = ¤ ¥ <br />

¡ ¡<br />

¡ ¥ ¤ ¥<br />

(a) <br />

¡ ¡<br />

¡ ¥ ¡ ¡ StrucSys.<br />

¥ ¤<br />

(b) copy( <br />

¥<br />

¡<br />

¡ ¥ ¡ ) = ¤ ¡ <br />

¡ ¡<br />

¡ ¥ ¤ ¥<br />

¡ ¡ Dom(¡ ¡ (c) proc ).<br />

¡ .<br />

¡ . Then<br />

The pro<strong>of</strong> <strong>of</strong> Proposition 1 is given in Appendix B. ¨<br />

According to (a) <strong>of</strong> Proposition 1, a copy <strong>of</strong> a Sys-structure is again a Sys-structure. Item<br />

(b) states that this Sys-structure is minimal in the sense that it does not contain any<br />

information about objects that are not (indirectly) known by ¤ . By item (c) we know that<br />

that implies that the Sys-structure does not contain any information about the involved<br />

process object also.<br />

Sys-structures satisfying property (b) <strong>of</strong> Proposition 16 , are called minimal Sys-structures.<br />

They will later be used extensively to describe data object passing between the various<br />

process objects. The set <strong>of</strong> all minimal Sys-structures StrucSys£ is denoted min. So, if <br />

¡ ¡<br />

¥ ¤ ¥<br />

StrucSys. Then<br />

¡<br />

<br />

¡ ¡<br />

¥ ¡ StrucSys£ min if and only if copy( ¤ ¥ <br />

¡ ¡<br />

¥ ) = ¤ ¥ <br />

¡ ¡<br />

¥ ¥ ¤<br />

<br />

Next, we define a collection <strong>of</strong> functions relabel m. These functions are used to relabel<br />

all object identifiers <strong>of</strong> a minimal Sys-structure by increasing them by m. Let <br />

¤ ¡ ¡ ¥ ¥<br />

¡¢<br />

StrucSys£ StrucSys£ £<br />

be a minimal Sys-structure, and let m be a natural number. Then the collection <strong>of</strong><br />

relabelling functions relabel m : min min is defined by<br />

where<br />

relabel m( <br />

¥<br />

¡ <br />

) ¤ ¡ ¥ ¡ ¡<br />

¡ ¥ ¡ <br />

¥ ¡ ¤<br />

6 Note that if © is a Sys-structure satisfying property (b), it also satisfies properties (a) and (c).

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