if token ="+"return plusnode( ( ), ( ) if token ="*"return timesnode( ( ), ( ) return numnode(float(token) def main() input("please enter prefix expression" lst split( queue queue( for token in lstq enqueue(token root ( print(root eval()print(root inorder() if __name__ ="__main__"main(in sect the parameter is queue of the tokens read from the file or string code to call this function is provided in the main function of sect the main function gets string from the user and enqueues all the tokens in the string (tokens must be separated by spaceson queue of tokens then the queue is passed to the function this function is based on the grammar given above the function looks at the next token and decides which rule to apply each call to the function returns an abstract syntax tree calling from the main function results in parsing the prefix expression and building its corresponding tree this example gives you little insight into how python reads program and constructs an abstract syntax tree for it python program is parsed according to grammar and an abstract syntax tree is constructed from the program the python interpreter then interprets the program by traversing the tree this parser in sect is called top-down parser not all parsers are constructed this way the prefix grammar presented in this text is grammar where the top-down parser construction will work in particulara grammar cannot have any left-recursive rules if we are to create top-down parser for it left recursive rules occur in the postfix grammar given in sect the postfix expression grammar ( ,ewhere {et {identifiernumber+* is defined by the set of productions number this copy belongs to 'acha