Unfamiliar syntax in function definition: function (arg1::type1)(arg2::type2)

I’m testing out an unregistered, abandoned package from 2020 (FEDVR.jl) because it looks like it might actually suit my research needs. I dev’d it locally, tossed the old manifest file (so it would use the latest versions of its dependencies rather than pinning them to their 2020 versions), reran the tests (they all passed) and got to work. Poking through the source code, there’s a few functions defined with a very odd syntax, found on lines 56-92 of basis.jl: function (basis::Basis)(x::Abstra...