let SAS be Semi_Affine_Space; :: thesis: for a, b, c, d being Element of SAS st parallelogram a,b,c,d holds
parallelogram a,c,b,d
let a, b, c, d be Element of SAS; :: thesis: ( parallelogram a,b,c,d implies parallelogram a,c,b,d )
assume parallelogram a,b,c,d ; :: thesis: parallelogram a,c,b,d
then ( not a,c,b is_collinear & a,b // c,d & a,c // b,d ) by Def3, Th56;
hence parallelogram a,c,b,d by Def3; :: thesis: verum