[Groff] pic: how to fill a diamond shape?

[Louis Guillaume]

Hi,

I'm making a flowchart with pic and I want to fill a "decision" diamond, 
which is drawn like this...

define decision {
  DECA: box invis wid $1 ht $2 $3
  line from DECA.w to DECA.n then to DECA.e then to DECA.s then to DECA.w
}

... is there a better way to draw the diamond? Or perhaps close the path 
made by the four lines so that it can be filled?

Louis