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