yacc grammar for a simple shell

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%token  STRING%token  RIGHT_APPEND%%cmd:    fg_cmd   |    bg_cmd   ;bg_cmd: fg_cmd '&'      ;fg_cmd:    simple_cmd      |    pipe_cmd      ;simple_cmd:     /* empty command */          |     executable          |     executable input_redirection          |     input_redirection executable          |     executable output_redirection          |     output_redirection executable          |     executable input_redirection   output_redirection          |     input_redirection   executable  output_redirection          |     input_redirection   output_redirection  executable          |     executable  output_redirection   input_redirection          |     output_redirection   executable  input_redirection          |     output_redirection   input_redirection  executable          ;executable: STRING          | STRING executable;input_redirection:  '<' STRING;output_redirection:     overwrite_output_redirection                  |     append_output_redirection                  ;overwrite_output_redirection:   '>'  STRING ;append_output_redirection:      RIGHT_APPEND STRING ;pipe_cmd:   left_cmd '|' right_cmd        |   left_cmd '|' middle_cmd right_cmd        ;left_cmd:   executable        |   executable input_redirection        |   input_redirection executable         ;middle_cmd: executable '|'          | middle_cmd executable '|'          ;right_cmd:  executable           |  executable output_redirection         |  output_redirection executable          ;%%int main(int argc, char *argv[]){    yyparse();    return 0;}int yywrap(){    return 1;}int yyerror(char *msg){    printf("Error encountered: %s\n", msg);}