reserve i,j for Element of NAT,
  x,y,z for FinSequence of COMPLEX,
  c for Element of COMPLEX,
  R,R1,R2 for Element of i-tuples_on COMPLEX;

theorem Th30:
  for x,y,z being complex-valued FinSequence st len x=len y & len y=len z
  holds x - y - z = x - (y + z)
proof
  let x,y,z be complex-valued FinSequence;
  assume that
A1: len x=len y and
A2: len y=len z;
  len (-y) = len y & len (-z) = len z by Th5;
  then x - y - z = x + (- y + - z) by A1,A2,Th24
    .= x - (y + z) by A2,Th29;
  hence thesis;
end;
