mpolyList arithmetic with single mpoly

[dkahle]

library("mpoly")

mp(c("x", "y")) + mpolyList(mp("2"))
#> Error: e1 and e2 must have equal length.

mp(c("x", "y")) + 2
#> Error: e1 and e2 must have equal length.

mp(c("x", "y")) + mp("2")
#> Warning: Incompatible methods ("+.mpolyList", "+.mpoly") for "+"
#> Error in mp(c("x", "y")) + mp("2"): non-numeric argument to binary operator

^{Created on 2019-03-20 by the reprex package (v0.2.1)}

[dkahle]

See https://stackoverflow.com/questions/13659449/emulating-multiple-dispatch-using-s3-for-method-possible.