Represents a reference to a value. With a reference it is possible to get or set the
value. Passing around value references rather than the values themselves can avoid
incorrect duplication of operand evaluation. For example in 'list[index++]++' without
a value reference for 'list[index++]' it would be necessary to evaluate list[index++]
twice (once to get the value, once to determine where the value goes) and that would
double increment index.