reserve a, b, c, d, x, y, z for Complex;
reserve r for Real;

theorem
  angle(a,b,c) <> 0 implies angle(c,b,a) <> 0
proof
  assume angle(a,b,c) <> 0;
  then
A1: angle(a,b,c)+angle(c,b,a) = 2*PI by Th78;
  assume not thesis;
  hence contradiction by A1,Th68;
end;
