@generated function (σ::Sym)(X::Arg...)
    x = all(T <: ℇ for T ∈ X) ? (:(X...),) :
        Tuple(X[i] <: ℇ ? :(X[$i]) : :(ℇ(X[$i])) for i ∈ 1:length(X))
    quote
        ℯ = ℇ(σ, $(x...))
        ℯ isa ℇ{NoSort} ? throw(ArgumentError("$ℯ violates signature")) : ℯ
    end
end