Limit of defined function

Dear list members,

I have defined the function

disweib(x,a,b):=
   if not numberp(x) or not numberp(a) or not numberp(b)
      then 'disweib(x,a,b)
      else if a>0 and b>0 
              then if x>0
                      then 1-exp(-(b*x)^a)
                      else 0
              else error("disweib: illegal parameter/s")$


which is the Weibull distribution function, with positive parameters a and b. 
I'd like to have this function working together with the limit to get

(%i1) limit(disweib(x,3,2.1),x,inf);
(%o2)          1

but this is what Maxima really gives:

(%o2)        limit    disweib(x, 3, 2.1)
             x -> INF

I think it has somethig to do with the 'disweib(x,a,b) above, but I can't find 
the appropiate code to get the right answer.

Any help, please?

Mario
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Mario Rodriguez Riotorto
www.biomates.net