G.516 parallel planes

XI.def.8

Parallel planes are those which do not meet.
parallel_planes_meet_def

Required for

G.517, G.518, G.519, G.520, G.530, G.531, G.541, G.542, G.543, G.544, G.546, G.547, G.550, G.554, G.555, G.556, G.557, G.558, G.559, G.560, G.563, G.565, G.570, G.571, G.572, G.573, G.577, G.578