fprintf(stderr, "stack underflow in the parser\n");
av_log(NULL, AV_LOG_ERROR, "stack underflow in the parser\n");
static double pop(Parser *p){    if(p->stack_index<=0){        fprintf(stderr, "stack underflow in the parser\n");        return NAN;    }//printf("pop\n"); fflush(stdout);    return p->stack[ --p->stack_index ];}