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A general description is here.
S4 values are automatically coerced to vector (via ‘as.vector’) when subassigned into atomic vectors.
‘[<-’ with an S4 value into a list currently embeds the S4 object into its own list such that the end result is roughly equivalent to using ‘[[<-’. That behavior is deprecated. In the future, the S4 value will be coerced to a list with ‘as.list’.