take { the chordal edgeless _Graph} ; :: thesis: ( not { the chordal edgeless _Graph} is empty & { the chordal edgeless _Graph} is edgeless & { the chordal edgeless _Graph} is chordal )
thus ( not { the chordal edgeless _Graph} is empty & { the chordal edgeless _Graph} is edgeless & { the chordal edgeless _Graph} is chordal ) ; :: thesis: verum