Truncate a polynomial expression up to a given order using SymPy in Julia

using SymPy

# Keep only terms of a given orders in a polynomial expression
function truncate_upto(ex::Sym, x::Sym, order::Int)
    if ex.func.__name__ != "Add" 
	error("The main expression should be an addition") 
    end
    return ex.func(ex.args[findall(y->degree(y,x) ≤ order,ex.args)]...)
end