reserve f for Function;
reserve n,k,n1 for Element of NAT;
reserve r,p for Complex;
reserve x,y for set;
reserve seq,seq1,seq2,seq3,seq9,seq19 for Complex_Sequence;

theorem
  seq1 + (seq2 - seq3) = seq1 + seq2 - seq3
proof
  thus seq1 + (seq2-seq3) = seq1 + (seq2+-seq3) .= seq1+seq2-seq3 by Th7;
end;
