Executing function only if another function is defined

[carlmarcos]

The following function appends additional text to the `docstring' of the 
function `toucan'.

Because it may be possible that some functions are not available (e.g. during 
testing or debugging phases), I would like to test whether the function 
`toucan' has been defined before I call `docboost'.  What can I do? 

(defun toucan-usage ()
  "TODO"
   
  (docboost 'toucan "Usage Information"))