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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Calling defun within the body of a function is dangerous, especially when CodeWars does not tell you what warnings SBCL printed during compilation.
This solution has linear space usage.
Why do you use hash tables, if your keys are each positive integer in the range?
This comment is hidden because it contains spoiler information about the solution
Excluding reals and complexes is probably overly 'defensive'; the instructions specify that the argument is an integer.
It is clever, although arguably problematic... naive compilers might not carry over any optimisations, if they match based off the symbol name.