code: def do_OP_SUBSTR(vm):
      
      pos = vm.pop_nonnegative()
      length = vm.pop_nonnegative()
      vm.append(vm.pop()[length:length+pos])
docs: >>> s = [b'abcdef', b'\3', b'\2']
      >>> do_OP_SUBSTR(s, require_minimal=True)
      >>> print(s)
      [b'de']