6. Generate a YACC specification to recognize a valid arithmetic expression that uses operators +, – , *,/ and parenthesis.

Below is a YACC specification for recognizing valid arithmetic expressions that involve operators (+, -, *, /) and parentheses. I'll explain each line of the code afterward:

```yacc

%{

#include <stdio.h>

%}

%token NUMBER

%left '+' '-'

%left '*' '/'

%%

expression: expression '+' expression

           | expression '-' expression

           | expression '*' expression

           | expression '/' expression

           | '(' expression ')'

           | NUMBER

           ;

%%

int yylex() {

    // Code for tokenizing input and returning tokens

}

void yyerror(const char *s) {

    fprintf(stderr, "Error: %s\n", s);

}

int main() {

    yyparse();

    return 0;

}

```

Now, let's break down the YACC specification line by line:

1. `%{` ... `%}`: This is the C code section where you can include header files or declare global variables and functions.

2. `%token NUMBER`: This line defines the token name `NUMBER` which is used to represent numeric values in the grammar.

3. `%left '+' '-'`: This defines the precedence and associativity of the operators `+` and `-`. The `%left` declaration specifies that these operators are left-associative.

4. `%left '*' '/'`: This defines the precedence and associativity of the operators `*` and `/`. Similarly, they are also left-associative.

5. `expression: ... ;`: This section defines the grammar rules for valid expressions. It states that an expression can be formed by either two expressions separated by an operator, enclosed in parentheses, or just a single NUMBER token.

6. `int yylex() { ... }`: This function is used to tokenize the input. You should replace this with actual code to return tokens.

7. `void yyerror(const char *s) { ... }`: This function is called when an error is encountered during parsing. It's used to report parsing errors to the user.

8. `int main() { ... }`: This is the main function of the YACC program. It calls `yyparse()` to initiate the parsing process.

The grammar rules are defined using BNF-like syntax. For instance, `expression: expression '+' expression` means that an expression can be formed by two expressions separated by the `+` operator. Similarly, the other rules cover the different possible forms of arithmetic expressions.

When this YACC specification is used with a Lex lexer, you will be able to parse and recognize valid arithmetic expressions that use the specified operators and parentheses. Make sure to replace the placeholder tokenization code with actual code to tokenize the input properly.