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A general description is here.
Printing lists, pairlists or attributes containing calls with S3 class no longer evaluate those.
Printing S4 objects within lists and pairlists dispatches with ‘show()’ rather than ‘print()’, as with auto-printing.
The indexing tags (names or ‘[[<n>]]’) of recursive data structures are now printed correctly in complex cases.
Arguments supplied to ‘print()’ are now properly forwarded to print methods when printing lists, pairlists or attributes containing S3 objects.
The print parameters are now preserved when printing S3 objects or deparsing symbols and calls. Previously, printing lists containing S3 objects or expressions would reset these parameters.