The following code fails in 'Evaluate' with:
"This expression was expected to have type Complex but here has type double list"
Am I breaking some rule on operator over-loading on '(+)'?
Things are OK if I change '(+)' to 'Add'.

open Microsoft.FSharp.Math

/// real power series [kn; ...; k0] => kn*S^n + ... + k0*S^0
type Powers = double List

let (+) (ls:Powers) (rs:Powers) =
    let rec AddReversed (ls:Powers) (rs:Powers) =
        match ( ls, rs ) with
        | ( l::ltail, r::rtail ) -> ( l + r ) :: AddReversed ltail rtail
        | ([], _) -> rs
        | (_, []) -> ls
    ( AddReversed ( ls |> List.rev ) ( rs |> List.rev) ) |> List.rev

let Evaluate (ks:Powers) ( value:Complex ) =
    ks |> List.fold (fun (acc:Complex) (k:double)-> acc * value +  Complex.Create(k, 0.0)  ) Complex.Zero