theorem Th37:
  [:X1,X2,X3,X4:] = [:[:[:X1,X2:],X3:],X4:]
proof
  thus [:X1,X2,X3,X4:] = [:[:X1,X2,X3:],X4:] by ZFMISC_1:def 4
    .= [:[:[:X1,X2:],X3:],X4:] by ZFMISC_1:def 3;
end;
