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

theorem
  angle(a,b,c) = PI implies angle(c,b,a) = PI
proof
  assume
A1: angle(a,b,c) = PI;
  then angle(a,b,c)+angle(c,b,a) = 0+2*PI by Th78,COMPTRIG:5;
  hence thesis by A1;
end;
