[1 of 1] Compiling Main             ( lib_proc_lists.hs, interpreted )

lib_proc_lists.hs:6:57:
    Could not deduce (Num a) from the context (Ord a)
      arising from the literal `0' at lib_proc_lists.hs:6:57
    Possible fix:
      add (Num a) to the type signature(s) for `compare_population'
    In the second argument of `(>)', namely `0'
    In the first argument of `population', namely `(> 0)'
    In the second argument of `(<=)', namely `(population ((> 0)) b)'
Failed, modules loaded: none.