theorem Th31:
  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
A1: len x=len y & len y=len z;
  len (-z) = len z by Th5;
  hence thesis by A1,Th24;
end;
