[Pdfedit-support] transparent-filled rectangle

[roberto f. <foi...@gm...>]

I want to place a box around a formula in my PDF, so i tried to use the
rectangle tool, but this seen to be forced to have a fill color which is not
transparent.
how can I produce an empty rectangle? (barring the possibility to draw four
straight lines each time i need a rectangle :) )
ciao
roberto