>My idea was opposite. Draw the line surface pit as one line in one of the scraps and add option -clip off. And join walls of another scraps to that line (surface pit).

Ah ha, that has possibilities, and would work – tricky where each of the entrances has three overlying lines though.

(wall:invisible, ceiling-step, wall:pit –outline none)  It becomes computer programming more than drawing to get the lines to overlie each other perfectly.

Bruce