Mock functions in a package.Source:
Mocking allows you to temporary replace the implementation of functions within a package, which useful for testing code that relies on functions that are slow, have unintended side effects or access resources that may not be available when testing.
This works by using some C code to temporarily modify the mocked function in place. On exit, all functions are restored to their previous state. This is somewhat abusive of R's internals so use with care. In particular, functions in base packages cannot be mocked; to work aroud you'll need to make a wrapper function in your own package..
named parameters redefine mocked functions, unnamed parameters will be evaluated after mocking the functions
the environment in which to patch the functions, defaults to the top-level environment. A character is interpreted as package name.
Environment in which to add exit hander. For expert use only.
Suraj Gupta (2012): How R Searches And Finds Stuff