+(a::List...) is precursor for method ambiguity

[rleegates]

...which includes +() such that it immediately becomes ambiguous with a definition of +(a::SomeOtherType...) somewhere else (in my case I encountered this conflict while using both MultiPoly and Lazy). See the discussion on julia-users.

[MikeInnes]

Yup, good catch! Happy to take a PR to fix this, if you're up for it.