Question: Assigning vectors and matrices

This seems such a simple/basic question I'm almost too embarassed to ask.

Anyway. this is causing me some headaches

 

> A := <0|0>;

         A:= [0 0 ]

> B := A:

> A(1,1) := 2 ;

         A:= [2 0 ]

> B;

         B:= [2 0 ]

What do I do to prevent the elements of B changing if A changes, after using the assignment B:=A (or should I not be using this assignment?)  I mean, I would like the same behaviour as 

> a := 0;

         a:=0

> b := a;

         b:=0

> a := 2;

         a:=2

> b;

         b:=0

which seems to work as I "expect"...

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