Limitations of a two-pass assembler

I've come to realize that supporting foward references in a two-pass assembler [1] isn't always easy. The simple case of forward references I support:

	lda	#alpha
alpha	equ	5

On pass 1, alpha isn't defined, but by pass two, we have its value—5.

With this code, however:

	lda	#alpha
alpha	equ	bravo
bravo	equ	5

alpha is undefined on line 1, and it remains undefined even on line 2 because we haven't defined bravo yet. Thus when we end pass 1, alpha is still undefined. That it took me two years to even stumble across this issue is a bit surprising to me. I just haven't written 6809 assembly code like this.

Can I fix this? If I add another pass, probably. If I don't want to add another pass … I don't know. I would have to track expressions that aren't fully defined in pass 1, which could be a lot of work for an issue that might not come up all that often (if my own code is to go by). I mean, things can get quite pathological:

		lda	#Alpha
Alpha 		equ	Bravo+1
Bravo           equ	Charlie+1
Charlie		equ	Delta+1
Delta		equ	Echo+1
Echo		equ	Foxtrot+1
Foxtrot		equ	Golf+1
Golf		equ	Hotel+1
Hotel		equ	India+1
India		equ	Juliet+1
Juliet		equ	Kilo+1
Kilo		equ	Lima+1
Lima		equ	Mike+1
Mike		equ	November+1
November	equ	Oscar+1
Oscar		equ	Papa+1
Papa		equ	Quebec+1
Quebec		equ	Romeo+1
Romeo		equ	Sierra+1
Sierra		equ	Tango+1
Tango		equ	Uniform+1
Uniform		equ	Victor+1
Victor		equ	Whiskey+1
Whiskey		equ	Xray+1
Xray		equ	Yankee+1
Yankee		equ	Zulu+1
Zulu 		equ	1

lsawm (part of LWTools [2]) does properly handle this pathological case but it does six passes, not two. The other 6809 assembler I have, an older one written back in the 90s, doesn't and issues deceptive error messages, so it's not like I'm the only one to not handle this properly.

As of now, I just issue an error and let the programmer deal with it.