Avoid variables appearing in the exponent position

[Alexia369]

How to avoid 0.9735712^x0, where x0 appears at the exponent

[MilesCranmer]

You could do it with

function my_loss(tree, dataset::Dataset{T,L}, options)::L where {T,L}
    index_of_exp = 1 # if its the first operator in your list etc
    has_x_in_exp = any(tree) do node
        if node.degree == 1 && node.op == index_of_exp
            subnode = node.l
            return any(subnode) do n
                return n.degree == 0 && !n.constant
            end
        end
        return false
    end
    has_x_in_exp && return L(10_000_000_000)
    prediction, flag = eval_tree_array(tree, dataset.X, options)
    if !flag
        return L(10_000_000)
    end
    return sum(i -> (prediction[i] - dataset.y[i]) ^ 2, eachindex(dataset.y)) / dataset.n
end

[MilesCranmer]

Also see https://astroautomata.com/PySR/api/#working-with-complexities