reserve AS for AffinSpace;
reserve a,a9,b,b9,c,d,o,p,q,r,s,x,y,z,t,u,w for Element of AS;
reserve A,C,D,K for Subset of AS;

theorem
  ( A // C & C // D or A // C & D // C or C // A & C // D or C // A & D
  // C ) implies A // D by Lm7;
