Question: Understanding "evalb" and "assume" and "is"

Hi,

I'm just playing about with Maple's functions to see what they do. I'm confused by the behaviour of "evalb". When I enter:

 > evalb( (x=5 or x=7)  implies (x^2 = 49));
                                                                    true
 

Why is this problem evaluated to true?

Also, if I type the query:

> assume(z>2, y>z*z); is(z*z>4); is(y>z*z); is(y>4);
                                                                    true

                                                                    true

                                                                    false

I would expect the last query to return true not false. What is the logic being used? I see that if I ask:

> assume(u>4,y>u); is(y>4);
                                                                    true

the correct answer is given. Why?

Regards,

Laura