embody all the operatorsso while it would be harmless to use list here--and might be essential for other parsers--in this example it isn' necessary def p_formula_equals( )"formula term equals termp[ [ [ ] [ ] [ ]this is the part of the bnf that relates formulas and terms the implementation is straightforwardand we could have included this with the other binary operators since the function' suite is the same we chose to handle this separately purely because it is logically different from the other binary operators def p_term( )"""term symbol lparen termlist rparen symbol"" [ [ if len( = else [ [ ] [ ]def p_termlist( )"""termlist term comma termlist term"" [ [ if len( = else [ [ ] [ ]terms can either be single symbol or symbol followed by parenthesized term list ( comma-separated list of terms)and these two functions between them handle both cases def p_error( )if is noneraise valueerror("unknown error"raise valueerror("syntax errorline { }{ }formatp lineno type)if parser error occurs the p_error(function is called although we have treated the ply yacc yaccproduction argument as sequence up to nowit also has attributesand here we have used the lineno attribute to indicate where the problem occurred precedence (("nonassoc""forall""exists")("right""implies")("left""or")("left""and")("right""not")("nonassoc""equals")to set the precedences and associativities of operators in ply parserwe must create precedence variable and give it list of tuples where each tuple' first item is the required associativity and where each tuple' second and subsequent items are the tokens concerned ply will honor the specified as