Say I have 4 beams which make a rectangle.
I pull away the top beam and then click on the face of ONE of the revealed faces on the side beams and move it extending the length of one side.
If I could put the top beam back, I'd now have a trapezium.
I can move the top beam up to meet the top of the longer size, but how then do I rotate it so that the other end of the top beam meets the shorter side?