Information Technology Reference
In-Depth Information
Figure 2.6.
A subway knowledge base
bay is left of yonge.
st-george is left of bay.
spadina is left of st-george.
bathurst is left of spadina.
christie is left of bathurst.
If X is left of Y then X is west of Y.
If X is left of Y and Y is west of Z then X is west of Z.
If X is west of Y then Y is east of X.
2.
Explain informally why the sentence
spadina is right of bathurst
is
not
entailed by
the knowledge base.
3.
Trace the back-chaining procedure on the following queries:
a.
spadina is west of st-george
;
b.
yonge is east of bay
;
c.
christie is west of spadina
;
d.
yonge is west of yonge
;
e.
st-george is east of bathurst
;
f.
bay is west of sherbourne
.
4.
Suppose the second conditional sentence in the knowledge base is replaced by
If X is west of Y and Y is left of Z then X is west of Z.
Would this change what is entailed? What happens now with the back-chaining
procedure on the query (3d)
yonge is west of yonge
?
5.
Suppose the (incorrect) atomic sentence
yonge is left of bay.
were added to the original knowledge base. For what values of
X
would the
resulting knowledge base entail
spadina is west of X
? What happens now with
the back-chaining procedure on the query (3f)
bay is west of sherbourne
?
