Question: How to obtain the new user-assigned names?

Suppose you do this

save something, afile.m;

and that later someone else does this

read afile.m;

Let's assume that this person does not know the names of the variables you have saved.
A way to get these names could be

before := { anames('user') };
read file.m:
after := anames('user') minus before minus {'before'}

I thought after would only contain something, but it also contains all the elements of before, just as if  
anames('user') minus before was not effective (see PS below)
Exemple

restart:
interface(version)
Standard Worksheet Interface, Maple 2015.2, Mac OS X, December 

   21 2015 Build ID 1097895
a:=1:
before := {anames('user')};
                              {a}
b:=2:
after := {anames('user')} minus before minus {'before'}
                         {a, b}

How can I obtain the set of the new user-asigned names?

PS: Why does displaying  before give the value assigned to a and not a itself?

before
                              {1}

Thanks in advance