cycle units measurement through hotkey is it possible? i find imperial is much easy or convenient to setup with camera distance from 
object a to b and eye level height 

i want to set a hotkey that cycle through units ( metric , imperial ) 
hitting same hotkey instead of manually going to tab and click the desired units button 

i tried to use wm.context_toggle_enum and type the following attributes but gives error
context.scene.system , scene.system , UnitSettings.system , data.scene , data.scene.system
or do i need a custom .py script in order to achieve the cycling of metric and imperial units

thank you for answer
The path to use for the context Attribute is

scene.unit_settings.system


Where you find a class name of UnitSettings is shown with it's name in CamelCase, you will often find a corresponding property that represents it will use the form unit_settings with all lowercase and underscore separating words.