theorem Th38:
  [:[:X1,X2:],X3,X4:] = [:X1,X2,X3,X4:]
proof
  thus [:[:X1,X2:],X3,X4:] = [:[:[:X1,X2:],X3:],X4:] by ZFMISC_1:def 3
    .= [:X1,X2,X3,X4:] by Th37;
end;
