 the lvalue of <tt>x</tt>, in 
this case it is an <tt>SVal</tt> that references the <tt>MemRegion</tt> for <tt>x</tt>. 
Afterwards, when we do the lvalue-to-rvalue conversion, we get a new <tt>SVal</tt>, 
which references the value <b>currently bound</b> to <tt>x</tt>. That value is 
symbolic; it's whatever <tt>x</tt> was bound to at the start of the function. 
Let's call that symbol <tt>$0</tt>. Similarly, we evaluate the expression for <tt>2</tt>, 
and get an <tt>SVal</tt> that references the concrete num