The set of tokens of the language is {+,*,NUMBER}. The token NUMBER represents unsigned integers, i.e., non-empty sequences of digits. Recall that 4, 2*4*5, 1+2*3*4, 1*2+3*4+5 are correct but *, 6+, +*, 1 2, *3, 1*2*3* are not. The generated AST must have at most one symbol + that will be the root, and only one * for every product. The usual precedence of * over + applies. For example, for input 1+2*3*4+5 the resulting AST must be +(1,*(2,3,4),5), whereas 1 has AST 1 and 1*2*3 has AST *(1,2,3).