 As a simple example of abstract terms consider linear polynomials of the form \(ax + by\) where \(a\) and \(b\) are nonnegative integers, for example \(7x + 3y\). The set of all such polynomials includes 0 and is closed under polynomial addition, an associative and commutative operation. This set together with the operation of addition and the zero polynomial therefore constitutes a commutative monoid.