static AVEvalExpr * parse_primary(Parser *p) {
    AVEvalExpr * d = av_mallocz(sizeof(AVEvalExpr));
    char *next= p->s;
    int i;

    /* number */
    d->value = av_strtod(p->s, &next);
    if(next != p->s){
        d->type = e_value;
        p->s= next;
        return d;
    }
    d->value = 1;

    /* named constants */
    for(i=0; p->const_name && p->const_name[i]; i++){
        if(strmatch(p->s, p->const_name[i])){
            p->s+= strlen(p->const_name[i]);
            d->type = e_const;
            d->a.const_index = i;
            return d;
        }
    }

    p->s= strchr(p->s, '(');
    if(p->s==NULL){
        *p->error = "undefined constant or missing (";
        p->s= next;
        ff_eval_free(d);
        return NULL;
    }
    p->s++; // "("
    if (*next == '(') { // special case do-nothing
        av_freep(&d);
        d = parse_expr(p);
        if(p->s[0] != ')'){
            *p->error = "missing )";
            ff_eval_free(d);
            return NULL;
        }
        p->s++; // ")"
        return d;
    }
    d->param[0] = parse_expr(p);
    if(p->s[0]== ','){
        p->s++; // ","
        d->param[1] = parse_expr(p);
    }
    if(p->s[0] != ')'){
        *p->error = "missing )";
        ff_eval_free(d);
        return NULL;
    }
    p->s++; // ")"

    d->type = e_func0;
         if( strmatch(next, "sinh"  ) ) d->a.func0 = sinh;
    else if( strmatch(next, "cosh"  ) ) d->a.func0 = cosh;
    else if( strmatch(next, "tanh"  ) ) d->a.func0 = tanh;
    else if( strmatch(next, "sin"   ) ) d->a.func0 = sin;
    else if( strmatch(next, "cos"   ) ) d->a.func0 = cos;
    else if( strmatch(next, "tan"   ) ) d->a.func0 = tan;
    else if( strmatch(next, "atan"  ) ) d->a.func0 = atan;
    else if( strmatch(next, "asin"  ) ) d->a.func0 = asin;
    else if( strmatch(next, "acos"  ) ) d->a.func0 = acos;
    else if( strmatch(next, "exp"   ) ) d->a.func0 = exp;
    else if( strmatch(next, "log"   ) ) d->a.func0 = log;
    else if( strmatch(next, "abs"   ) ) d->a.func0 = fabs;
    else if( strmatch(next, "squish") ) d->type = e_squish;
    else if( strmatch(next, "gauss" ) ) d->type = e_gauss;
    else if( strmatch(next, "mod"   ) ) d->type = e_mod;
    else if( strmatch(next, "max"   ) ) d->type = e_max;
    else if( strmatch(next, "min"   ) ) d->type = e_min;
    else if( strmatch(next, "eq"    ) ) d->type = e_eq;
    else if( strmatch(next, "gte"   ) ) d->type = e_gte;
    else if( strmatch(next, "gt"    ) ) d->type = e_gt;
    else if( strmatch(next, "lte"   ) ) { AVEvalExpr * tmp = d->param[1]; d->param[1] = d->param[0]; d->param[0] = tmp; d->type = e_gt; }
    else if( strmatch(next, "lt"    ) ) { AVEvalExpr * tmp = d->param[1]; d->param[1] = d->param[0]; d->param[0] = tmp; d->type = e_gte; }
    else if( strmatch(next, "ld"    ) ) d->type = e_ld;
    else if( strmatch(next, "st"    ) ) d->type = e_st;
    else if( strmatch(next, "while" ) ) d->type = e_while;
    else {
        for(i=0; p->func1_name && p->func1_name[i]; i++){
            if(strmatch(next, p->func1_name[i])){
                d->a.func1 = p->func1[i];
                d->type = e_func1;
                return d;
            }
        }

        for(i=0; p->func2_name && p->func2_name[i]; i++){
            if(strmatch(next, p->func2_name[i])){
                d->a.func2 = p->func2[i];
                d->type = e_func2;
                return d;
            }
        }

        *p->error = "unknown function";
        ff_eval_free(d);
        return NULL;
    }

    return d;
}