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a = [5,5,5]
b = [5].each.with_object( a ) { |del| a.delete_at( a.index( del ) ) }
b = [5,5]

# AS A LAMBDA

subtract = lambda do |minuend, subtrahend|
  subtrahend.each.with_object( minuend ) { |del| minuend.delete_at( minuend.index( del ) ) }
end

subtract.call a, [5]
# return [5,5]