def to_s
    out = ""

    @v.each do |line|
      out << "["

      # display all elements of this line
      line.each do |element|
        out << element.to_f.round(3).to_s << " "
      end

      out << "\b]\n" # TODO : FIX THAT broggi_t
    end
    out
  end
def to_s
    out = ""

    @v.each do |line|
      out << "["

      # display all elements of this line
      line.each do |element|
        out << element.to_f.round(3).to_s << " "
      end

      out << "\b]\n" # TODO : FIX THAT broggi_t
    end
    out
  end
== Parameters:
 tab::
   tab is a double Array like [ [1,2], [3,4], [1,4] ]. This array should be a valid Matrix tab or an other Matrix

 == Returns:
   nothing