Human-Computer Interaction 3e Dix, Finlay, Abowd, Beale

In Section 17.2.5, we said that the specification Algebraic-draw could be extended to say that a move, resize or unselect after a delete has no effect. The axiom for unselect looks like this:

(9) *unselect*(*delete*(*g*))
= *delete*(*g*)

Write two more axioms (10) and (11) which say the same about move and resize. Now use axioms (4) and (5) to show that (9) implies both your new axioms.

*answer*

The three additional axioms are:

(9) *unselect*(*delete*(*st*))
= *delete*(*st*)

(10) *move*(*p*,*delete*(*st*))
= *delete*(*st*)

(11) *resize*(*p*,*delete*(*st*))
= *delete*(*st*)

Axioms 4 and 5 are:

(4) *move*(*p*,*unselect*(*st*))
= *unselect*(*g*)

(5) *resize*(*p*,*unselect*(*st*))
= *unselect*(*g*)

To show that Axiom 10 is derivable from Axioms 4 and 9 we can argue as follows:

move(p,delete(st)) | |||

= move(p,unselect(delete(st)) |
by Axiom 9 | ||

= unselect(delete(st)) |
by Axiom 4 | ||

= delete(st) |
by Axiom 9 |

A similar argument shows that Axiom 11 is derivable from Axioms 5 and 9.

**Other exercises in this chapter**

ex.17.1 (ans), ex.17.2 (ans), ex.17.3 (ans), ex.17.4 (ans), ex.17.5 (tut), ex.17.6 (tut), ex.17.7 (tut), ex.17.8 (tut), ex.17.9 (tut)

all exercises for this chapter