reserve AFV for WeakAffVect;
reserve a,b,c,d,e,f,a9,b9,c9,d9,f9,p,q,r,o,x99 for Element of AFV;

theorem Th21:
  ex c st Mid a,b,c
proof
  consider c such that
A1: a,b // b,c by Def1;
  Mid a,b,c by A1;
  hence thesis;
end;
