function sum(a,b,c,d)          -- "sum" method
  return a+b+c+d   -- return sum
end
print( sum( 1, 2, 3, 4 ) )
print( sum( 5, 6, 7 ) )
print( sum( 9, 10, 11, 12, 13, 14 ) )
print( sum( sum(1,2,3,4), sum(5,6,7), sum(9,10,11,12,13,14), 15 ) )