In a procedure what is the way to only allow specific values in a variable?

ie/

change:=proc(a::integer, b={10,20,30})#only values 10, 20 and 30 allowed for variable b
  a*b;
end proc:

Trying to find a formula such that when n is negative the value is +1 and when n is positive the value is 0, without using an if statement. 

I was trying to find the best way to determine what powers of 10 a number is. 

trunc(log10(a)) #a being any number

It works great for integers, however it fails when the number has is some integer with 7 nine's in the decimal.
trunc(log10(99.999999))
                                                  1

trunc(log10(99.9999998))
                                                 1

trunc(log10(99.9999999))
                                                 2

To get around that fail I found length to be another way. 
     length(trunc(a))-1  #a is any number

Any other ways or mayber there's a better way?

Where is the backup directory of auto saves stored - windows?

This is probaby more of a software request and I think it would be useful. 

You can obviously open two worksheets of maple and tile them vertically to give the effect of a split screen but then all the icons of each worksheet reduce the visible on screen real estate in which you can work.

It should work similar to how Excel does it when you split cells.  It would work nicely in situations where you have a diagram or picture you are referring to during the creation of your worksheet. 

Anyways just a request, I'm sure some people would find that functionality welcome.