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A general description is here.
The ‘@<-’ operator is now implemented as a primitive, which should reduce some copying of objects when used. Note that the operator object must now be in base: don't try to import it explicitly from package "methods".
Field assignments in reference class objects via ‘$<-’ were not being checked because the magic incantation to turn methods on for that primitive operator had been inadvertently omitted.
‘dt(1e160, 1.2, log=TRUE)’ no longer gives ‘-Inf’.