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+3,6+3+8+1 are correct but +, 1+, ++, 1 2, +1 are not. The generated AST will have exactly one symbol sum that will be the root. For example, 1+2+3+4 has AST sum(1,2,3,4), and 1 has AST sum(1).