 
  layer on top of other regions, providing a layered approach to representing memory. 
  For example, a struct object on the stack might be represented by a <tt>VarRegion</tt>, 
  but a <tt>FieldRegion</tt> which is a subregion of the <tt>VarRegion</tt> could 
  be used to represent the memory associated with a specific field of that object.
  So how do we represent symbolic memory regions? That's what 
  <a href="http://clang.llvm.org/doxygen/classclang_1_1ento_1_1SymbolicRegion.html">SymbolicRegion</a> 