reserve n for Nat;

theorem Th5:
  for x,y be VECTOR of REAL-NS n, a,b be Element of REAL n st x=a &
  y=b holds x-y = a - b
proof
  let x,y be VECTOR of REAL-NS n;
  let a,b be Element of REAL n;
  assume that
A1: x=a and
A2: y=b;
  -y = -b by A2,Th4;
  hence thesis by A1,Th2;
end;
