A grammar consists of a collection of substitution rules, also called produc- tions. Each rule appears as a line in t he grammar, comprising a symbol and a string separated by an arrow. The symbol is c alled a variable. The s tring consists of variables and other symbols called terminals. T he variable symbols often a re represented by capit al letters. The terminals are analogous to the in- put alphabet and often are represented by lowercase letters, numbers, or special symbols. One variable is designated as the start variable. It usually occurs on the left-hand side of the topmost rule