It means that to be portable you have to wrap symbols that represent functions in (function <the-symbol>) when passing them as arguments. Not doing so will work

It means that to be portable you have to wrap symbols that represent functions in (function <the-symbol>) when passing them as arguments. Not doing so will work on some ports but not on others, and should be considered "undefined behaviour". Portable: (foldl (function +) 0 [1 2 3]) May work depending on the backend: (foldl + 0 [1 2 3])