is null if the access is empty, e.g. a[,3]
the accessed container/variable/expression
allows to attach additional information to the node
the original string retrieved from R, can be used for further identification
The location may differ from what is stated in Source#fullRange as it represents the location identified by the R parser.
Readonly
type
access can be a number, a variable or an expression that resolves to one, a filter etc.