theorem Th28:
  PSym(p,a) = a iff a = p or MDist a,p
proof
A1: now
    assume a = p or MDist a,p;
    then Mid a,p,a by Th18;
    hence PSym(p,a) = a by Def4;
  end;
  now
    assume PSym(p,a) = a;
    then Mid a,p,a by Def4;
    hence a = p or MDist a,p;
  end;
  hence thesis by A1;
end;
