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A general description is here.
approxfun() and approx() now accept a 'rule' of length two, for easy specification of different interpolation rules on left and right. They also no longer segfault for invalid zero-length specification of 'yleft, 'yright', or 'f'.
seq_along(x) is now equivalent to seq_len(length(x)); previously it was not, when x had a "surprising" length() method.