lpp10 = [ epp5[0] + lid_gap_x, kppc[1] + lid_gap_z ];
lpp11 = [ lpp10[0], epp5[1] + lid_gap_z ];
lpp10 = [ epp5[0] + lid_gap_x, kppc[1] + lid_gap_z ];
lpp11 = [ lpp10[0], epp5[1] + lid_gap_z ];
translate(skew_centre)
multmatrix([[ 1, 0, 0, 0 ],
translate(skew_centre)
multmatrix([[ 1, 0, 0, 0 ],