[galaxy-dev] en/disable field based on the state of checkbox/radiobutton

Hello, From looking at the documentation for tool config syntax and I wasn't able to spot a way to have a field enabled or disabled based on the state of a radio button or checkbox. Is this possible? If I missed the documentation please point me in the right direction. I have two use cases for this: the first is that I want the user to be able to either select from a group of preselected values (in my case genome lengths) or enter their own value in a numeric field; the second is that I'd like the user to be able to select from a data file that has been stored on our server or a data file from their history. Thank you, Keith