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, +3 are not. The generated AST will have at most one symbol + that will be the root. For example, 1+2+3+4 has AST +(1,2,3,4), whereas 1 has AST 1.