code: def find_include_file(self, t):
          
          fname = t[2]
          for d in self.searchpath[t[1]]:
              if d == os.curdir:
                  f = fname
              else:
                  f = os.path.join(d, fname)
              if os.path.isfile(f):
                  return f
          return None
docs: Finds the #include file for a given preprocessor tuple.