An issue common to both clapconf and clapenv is that Parse() also validates the input. When all we're dealing with is command-line args, this is fine, but when multiple engines can provide data, it causes a sort of race condition for mandatory arguments. If conf and env are loaded first, then we have an issue when the user tries to use command-line args that are also specified in (e.g.) environment variables: claptrap will throw a "too many arguments" error. If conv and env are loaded last, then the validation performed during Parse() happens before the values from conf and env are loaded.

Providing a non-validating Parse (say, claptrap.Load()) and exposing Validate would allow the tools to load any missing values and then call Validate themselves. In this case, we'd constrain the tools to be run after claptrap.Load(), and have them call the claptrap.Validate() when they're done:

claptrap.Load(nil)
clapenv.Load()