Comment by Nevor

Parent comment is referencing the "let inline" construction that in addition to inline code allows to constrain generic types structurally over member definition[1] instead of by name as usual. This gives us poor's man type classes which is useful nonetheless.

I guess the conclusion is that the definition of duck typing is fuzzy and misleading.

[1] https://msdn.microsoft.com/en-us/library/dd548046.aspx