Merge Shapes -- cycle through 'Scope' options

I want to assign a bindkey to cycle through the 'Merge Shapes' options.  In the documentation I only see a way to toggle 'Merge Shapes' on and off with 'mergeEnable' -- can't see a way to access the 'Scope' options of 'Pcells, Except current, All'.

I'd like to write a procedure to switch through 'mergeEnable nil' -> 'mergeEnable t w/ Pcells scope' -> 'mergeEnable t w/ Except current scope' -> 'mergeEnable t w/ All scope' -> back to 'mergeEnable nil'

Thanks.