[Asterisk-Users] Undocumented "exten" syntax?

[John Goerzen]

Over at http://www.voip-info.org/wiki-Asterisk+tips+911, I see these
extensions.conf lines:

exten => s,1,SetVar(SET_EMERG_FLAG=0)
exten => s,n(checkavail),ChanIsAvail(${EMERGENCY_TRUNK})
exten => s,n,SetGlobalVar(EMERGENCY=1)
exten => s,n,SetVar(SET_EMERG_FLAG=1)
exten => s,n(dial),Dial(${EMERGENCY_TRUNK}/${EMERGENCY_NUM})
exten => s,s+2(trunkbusy),GotoIf($[${EMERGENCY} = 1]?inprogress)

Now, I have several questions:

 * What is the "n" priority and how can they use it for several
   different items?  Don't they need an increasing integer there?

 * What is the "(checkavail)" doing?

 * What does "s+2" mean?

I've tried looking in docs and the wiki but can't figure it out.

Thanks!

-- John