What is the point of the 'endrule' attribute of smooth()'s return value?
For the simpler smooths (not involving 'S'?) it is value of the endrule
argument, for more complicated smooths it is the endrule not asked for (the
choices are 'Tukey' and 'copy'). For kind="S", it is not in the return
value, no matter what the input is.