Productions are the mechanism used to describe how complex input structures are built up out of simpler ones. Each production has a left side and a right side. The right side, or grammar rule, is a sequence of rule elements, which may represent either terminal tokens or nonterminal tokens. The left side is a list of reduction tokens. In most cases there would be only a single reduction token. Productions with more than one token on the left side are called semantically determined productions. The "->" symbol is used to separate the left side from the right side.