reserve X1,X2,X3,X4 for set;

theorem Thm05:
  union {X1,X2,X3} = X1 \/ X2 \/ X3
  proof
    thus union {X1,X2,X3} = union ({X1,X2} \/ {X3}) by ENUMSET1:3
        .= union {X1,X2} \/ union {X3} by ZFMISC_1:78
        .= X1 \/ X2 \/ union {X3} by ZFMISC_1:75
        .= X1 \/ X2 \/ X3;
  end;
