The set of tokens of the language is {+,NUMBER}. The token NUMBER represents unsigned integers, i.e., non-empty sequences of digits. Recall that 1,2+3,4+5+6+7 are correct but +, 1+, ++, 1 2, +5 are not. The generated AST must correspond to an interpretation of + as a right-associative operator. For example, for input 1+2+3+4 the resulting AST must be +(1,+(2,+(3,4))), i.e., like if the implicit parenthesization was 1+(2+(3+4)).