Thank all of you!

I solved my problems using the command type(). However the error I wrote before was because I didn't write 'evalf' in the if condition...If I write 

 ...

if evalf(Sol2)=NULL then

...

it works!

But it remains a mistery for me, why maple dosen't need the "evalf" commanda inside the type command. I can write

...

if (not type(sol2, numeric)) then

..

and it works fine!

I'm very wandered by the competence and velocity on answering in this forum, Thank you again!

Andrea