code: def validate(self):
          
  
          for function in compat_itervalues(self.functions):
              for callee_id in compat_keys(function.calls):
                  assert function.calls[callee_id].callee_id == callee_id
                  if callee_id not in self.functions:
                      sys.stderr.write('warning: call to undefined function %s from function %s\n' % (str(callee_id), function.name))
                      del function.calls[callee_id]
docs: Validate the edges.