In Example 2 the + sign is introduced, meant to represent addition of natural numbers. Its definition is almost identical to the previous one, with input and output sorts, but there is a difference: its operator has underscores on each side. Maude lets the user specify whether or not operators are infix, postfix or prefix (default), this is done using underscores as place fillers for the input terms. So the + operator takes its input on each side making it an infix operator. Unlike our previous operator s which takes its input terms after the operator (prefix).