This should be a hell of a lot simpler, just 

````shell

RVM_STRING="rvm use rbx-head@hatter --create --verbose"
[ -e './.rvmrc_local' ] && source './.rvmrc_local' || $RVM_STRING
````

That gives you the ability to set a .rvmrc_local if you want to override it, and will use rbx-head by default. You can replace that with whatever ruby you like, but I'd really love if Hatter was run on RBX.