Qualified identifier

These may occur in a list of identifiers to define subsets of the values of a data structure. The form is "identifier $ qualifier", where the qualifier is a sequence of identifier lists enclosed in square brackets. For factors, variates, and texts, the qualifier has a single list, each element of which defines a subset of the vector concerned. For matrices there are two lists running in parallel, one for each dimension. For a symmetric matrix, there can be either one or two lists, depending on whether or not its two dimensions are to be subsetted in the same way. For a diagonal matrix there is a single list. Tables cannot be qualified. The elements of the qualifier lists can be scalars, numbers, variates, quoted strings, or texts.