code: def activate(self):
          
          if self._handle != self._parser.env.get('current_program', False):
              self._parser.env['current_program'] = self._handle
              gl.glUseProgram(self._handle)
docs: Avoid overhead in calling glUseProgram with same arg.
          Warning: this will break if glUseProgram is used somewhere else.
          Per context we keep track of one current program.