What is the point in expression: !!var ?

[twilight]

Hello,

I'm digging through FreeBSD cppcheck scan report with hope to catch some
bugs.

I've came along a line:
	c = cond(token, val, yylex(), noeval | !!a);

What's the point in using !! twice? Can it be deleted?
Here is the context:

static arith_t cond(int token, union yystype *val, int op, int noeval)
{
	arith_t a = or(token, val, op, noeval);
	arith_t b;
	arith_t c;

	if (last_token != ARITH_QMARK)
		return a;

	b = assignment(yylex(), noeval | !a);

	if (last_token != ARITH_COLON)
		yyerror("expecting ':'");

	token = yylex();
	*val = yylval;

	c = cond(token, val, yylex(), noeval | !!a);

	return a ? b : c;
}


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