Trigger to call a function to get info to see if ok to fire trigger

I have been looking on forum for information on how to do this for 4 hours.

I found this
http://www.mushclient.com/forum/?id=6030

I tried this


<triggers>
  <trigger
   enabled="y"
   match="You are thirsty."
   script="my_stats"
   send_to="12"
   sequence="100"
  >
  <send>if cma &gt; 20 then

Send ("cast 'freshwater'")

endif</send>
  </trigger>
</triggers>
>


<triggers>
  <trigger
   enabled="y"
   match="&lt;*/*hp */*ma */*mv&gt;"
   script="my_stats"
   send_to="12"
   sequence="100"
  >
  </trigger>
</triggers>


function test_alias (name, line, wildcards)
  Note "Hello, world!"
end -- function test_alias


function my_stats (name,line,wildcards)
local chp = wildcards [1]
local mhp = wildcards [2]
local cma = wildcards [3]
local mma = wildcards [4]
local cmv = wildcards [5]
local mmv = wildcards [6]


end--function my_stats

How I wanted it to work was. It would see You are thirsty. It for check my_stats. It would see if I had 20 or more mana available. If the answer was yes, then it would cast the freshwater spell.

I get:


Error number: 0
Event:        Run-time error
Description:  [string "Trigger: "]:1: attempt to compare number with nil

stack traceback:

	[string "Trigger: "]:1: in main chunk
Called by:    Immediate execution