function P_TRIG(var, previous)
    -- detect rising or falling state of the register var
    -- false for nil input 
    -- previous regoster value stored in previous register (id or alias)
    DEBUG("P_TRIG was called with this " .. var .. " " ..previous)
 
    local curVar = R(var)       
    if not curVar then return false end 
 
    local prevVar = R(previous) ; DEBUG(" p_trig " .. curVar .. " " .. prevVar)
 
    W(previous, curVar)
 
    if (curVar ~= prevVar) then 
        if (curVar > prevVar) then 
            return "positive" 
        else 
            return "negative"
        end 
    end 
            return false -- nothing happened 
end -- P_TRIG 

-- usage example 
function main(userId)
    if P_TRIG("pulseId", "pulsePreviousValue") == "positive" then 
        local counterNewValue = R("counterId") + 1
        W("counterId", counterNewValue)
    end 
end 

-- Another version using global callable object
P_TRIG = setmetatable({},  -- callable objects like P_TRIG_FOR_COUNTER ... 
         {__call = function(self, regToTrack) 
                      -- init section 
                      local newValue = R(regToTrack)
                      if (not self.previousValue) then 
                          self.previousValue = newValue  
                          return false 
                      end 
                      -- main logic 
                      local result 
                      if (self.previousValue ~= newValue) then 
                          if (newValue > self.previousValue) then 
                              result = 'rise'
                          else 
                              result = 'fall'
                          end 
                      else 
                          result = false 
                      end 
                      self.previousValue = newValue -- store previous value 
                      return result
                  end } )

-- usage 
function main(userId)
    local ptrig = P_TRIG("testReg2") -- only one call in scan is correct!
    if (ptrig == "rise") then 
        DEBUG("rise detected!")
    elseif (ptrig == "fall") then 
        DEBUG("fall detected!")
    else 
        DEBUG("nothing detected!")
    end 
end