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Do not use parentheses when defining a function which has no arguments.

The code in this example ...

    def summer?() do
      # ...

... should be refactored to look like this:

    def summer? do
      # ...

Like all `Readability` issues, this one is not a technical concern.
But you can improve the odds of others reading and liking your code by making
it easier to follow.
No suggestions.
Please help! Open an issue on GitHub if this assessment is incorrect.