reserve a,b,i,j,k,l,m,n for Nat;

theorem
  2*n choose n = (2*n)!/((n!)|^2)
proof
  A1:n = (2*n) - n;
  n+n >= n+0 by XREAL_1:6; then
  2*n choose n = ((2*n)!) /((n!)*(n!)) by A1,NEWTON:def 3;
  hence thesis by NEWTON:81;
end;
