code: def is_child_of(self, parent):
          
          self.is_file()
          if not isinstance(parent, str_types):
              raise TypeError('given parent directory arg must be a path')
          val_abspath = os.path.abspath(self.val)
          parent_abspath = os.path.abspath(parent)
          if not val_abspath.startswith(parent_abspath):
              self._err('Expected file <%s> to be a child of <%s>, but was not.' % (val_abspath, parent_abspath))
          return self
docs: Asserts that val is an existing path to a file and that file is a child of parent.