describe('Monoid basics', () => {
  it('sums numbers including empty array', () => {
    expect(sumAllNumbers([1, 2, 3, 4, 5])).toBe(15)
    expect(sumAllNumbers([])).toBe(0) // Returns identity element
  })

  it('joins strings including empty array', () => {
    expect(joinStrings(['Hello', 'World'])).toBe('HelloWorld')
    expect(joinStrings([])).toBe('') // Returns identity element
  })

  it('handles single element', () => {
    expect(sumAllNumbers([42])).toBe(42)
    expect(joinStrings(['solo'])).toBe('solo')
  })
})