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unless Enumerable.method_defined? :sum
module Enumerable
# Sums up elements of a collection by invoking their `+` method.
# Most useful for summing up numbers.
#
# @param default [Object] an optional default return value if there are no elements.
# It's nil by default.
# @return The sum of the elements or the default value if there are no
# elements.
#
# @example
# [1, 2, 3].sum #=> 6
# ["a", "b", "c"].sum #=> "abc"
# [[1], [2], [3]].sum #=> [1, 2, 3]
# [].sum #=> nil
# [].sum(0) #=> 0
def sum(default = nil)
reduce(&:+) || default
end
end
end
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