Enumerables - how does Enum.reduce work with maps

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4 
alphabet_positions = {"a" => 1, "b" => 2, "c" => 3, ...}
Enum.reduce(alphabet_positions, 0, fn({letter, number}, accumulator) ->
  if "aeiou" =~ letter do
    accumulator += number
  end
  accumulator
end)

This trivial example adds up the positions of all the vowels in the alphabet.