ract, but named, symbolic value. Symbols represent 
  an actual (immutable) value. We might not know what its specific value is, but 
  we can associate constraints with that value as we analyze a path. For 
  example, we might record that the value of a symbol is greater than 
  <tt>0</tt>, etc.
  </p>

  <p>
  <a href="http://clang.llvm.org/doxygen/classclang_1_1ento_1_1MemRegion.html">MemRegion</a> is similar to a symbol.  
  It is used to provide a lexicon of how to describe abstract memory. Regions can