PyFile:SliceList.py
  PyExpressionStatement
    PySubscriptionExpression
      PyReferenceExpression: a
        PsiElement(Py:IDENTIFIER)('a')
      PsiElement(Py:LBRACKET)('[')
      PyTupleExpression
        PyReferenceExpression: b1
          PsiElement(Py:IDENTIFIER)('b1')
        PsiElement(Py:COMMA)(',')
        PySliceItem
          PsiElement(Py:COLON)(':')
      PsiElement(Py:RBRACKET)(']')