Whenable.Then(identifier)
args:
- identifier: a 
Stringkey identifier used to register one or more keyboard events with IsPressed(), IsReleased(), IsHeldFor() or IsInput() 
returns: Whenable
Then() is a stripped down version of When() that only accepts the identifier argument.
It's used to register a subsequent event in a Whenable chain for multi-key shortcuts where events other than pressed are required.
// creates a shortcut for pressing "a" then "b" consecutively with no time constraint
When('a').IsPressed()
  .Then('b').IsHeldFor(1).Seconds()
  .Then('a').IsReleased()
  .Execute(console.log);