Duh, sorry, ignore my explanation related to `param_dim` above. Non-linear fitting can only be done with one parameter set, an Error is raised otherwise. This is actually the output of a `tie` function, a mathematical relation between two parameters. So it is a scalar and `append` is appropriate there.
Well, most of the time ... Because if  a parameter's value is, for example, an array, this won't work. Although it's probably a corner case, I think we should support it. So `Parameter.parshape` is what matters here. And in addition, there's no guarantee that the `tie` function returns float values so `parameters._tofloat(), should probably be called. Sigh ....