undocumented key
i1 : f = method() o1 = f o1 : MethodFunction
i2 : f List := x -> 1 o2 = FunctionClosure[currentString:2:12-2:15] o2 : FunctionClosure
i3 : f VisibleList := x -> 2 o3 = FunctionClosure[currentString:3:19-3:22] o3 : FunctionClosure
i4 : f BasicList := x -> 3 o4 = FunctionClosure[currentString:4:17-4:20] o4 : FunctionClosure
i5 : undocumented { f, (f,List) }
The object undocumented is a method function with a single argument.