You're mixing two lines of thought, the former was me simply replying as the I think the person that tried to use Hit & Run might and why one line of reasoning against him is wrong.
The later is my personal opinion as to why I think he was wrong and I think the best reply is a question. Are you suggesting one can do things the BRB does not say we can do?
I even find your demand for a precedent strange, as discussions of RaW are not effected by precedent. If we were to use precedents then the first precedent would be, that they write rules/faq without regard precedent.
A little health now and again is the invalids best remedy.
Hit and Run only works when a unit is locked in combat, because it says "when a unit is locked in combat it may do this...". That is, if the condition of "being locked in combat" is fulfilled, then you may make the move. If that condition is not fulfilled, you may not. It doesn't need to be any more explicit than that. Under any real scrutiny, there's no argument this guy could make to back himself up.
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RaW does use precedents. One, for example, is the convention of 'order of operations'. Everything- a simple example: Space Marine Powerfist + Furious Charge. First double base strength, then add +1. Throughout the rules we apply an order of operations to what we read. In most cases, it is not formally written, but the one that I pointed out applies to almost all rules.
In fact, Darklink's last post is based upon the 'order of operations'. The rule first mentions 'locked in combat' then mentions 'may leave'.
Wait, are you trying to support your point or undermine it? Perhaps you're mixing optional (and thus irrelevant) and binding precedents? That you acknowledge exceptions implies to me you think they are not binding. Your example is of situation were anything else then a precise order of events would result in confusion even if most people defaulted to BEDMASE.
If precedents is the language you prefer to use then lets look at the precedent GW has set up in the rules. Every rule written makes clear which unit can do what and when they can do it. There are no exceptions to this. We might even call this a binding (through weight of consistency) precedent as there is no rule in the BRB + codexes that fails this who, what, when formulation. The order stated isn't relevant to anything other then readability.
So how does Hit & Run accomplish this?
Units locked in combat may do the what and when. It does not say "units lock in combat can" which would be synonymous with "only units locked in combat can". The OP's friend possibly read may and then took it as an non-binding requirement, as being locked in combat might be the case but need not be the case. I won't argue about whether that's the right way or not to read it. I think DarkLink's point is that that is the wrong way to read it. My point is that that is one of two ways to read it. Now if this person agrees with DarkLink it ends there. But if it doesn't end there and I know people that wouldn't let it end there. So if that sentence isn't the sentence that tells us explicitly the crucial "who" from who, what, when then which one does? Well, none of them do, and if none of them do then the rule does nothing.
DarkLink and I disagree on the why the OP's friend was wrong not on that he was wrong. Is this really a conversation worth having?
A little health now and again is the invalids best remedy.