_Symbols _with _leading _underscores

[Paul]

I see code like this from time to time.  Are the leading 
underscores significant, in general, in the D language?  Is it 
just programmer preference? Is it a coding practice, in general, 
that is common...even outside of D?  Thanks for any assistance.

From: 
http://dpldocs.info/this-week-in-d/Blog.Posted_2022_10_10.html#hello-arduino

```d
import ldc.llvmasm;

// Ports from the delay_basic.h in the thing
void _delay_loop_1(ubyte __count) {
	// this template param is required to avoid
	// assertion `!Call.getType()->isVoidTy() && "Bad inline asm!"' 
failed.

         __asm!ubyte (
                 "1: dec $0\n\tbrne 1b",
                 "=r,0", (__count)
         );
}
void _delay_loop_2(ushort __count) {
         __asm!ushort (`
	1: sbiw $0,1
	brne 1b
	`,
	`=w,0`,
	__count);
}

// port from delay.h in arduino thing
enum F_CPU = 1_000_000UL;

// this was _delay_ms but i did something wrong and changed 
double to int and i still don't love it
void _delay(int __ms) {
	ushort __ticks;
	ulong __tmp = (F_CPU * __ms) / 4000;
	if(__tmp < 1)
		__ticks = 1;
	else if(__tmp > 65_535) {
		__ticks = cast(ushort) (__ms * 10.0);
		while(__ticks) {
			_delay_loop_2(cast(ushort) (((F_CPU) / 4e3) / 10));
			__ticks--;
		}
		return;
	} else
		__ticks = cast(ushort) __tmp;
	_delay_loop_2(__ticks);
}
```