it('with setters', () => {
   const labelledToNum = mkSetter(
     (f: (label: string) => number) => (ln: Labelled<number>) => f(ln.label) + ln.x)
   const comp = lp.compose(labelledToNum)
   const strlen = (s: string) => s.length
   expect(comp.modify(strlen, left(7))).to.eql('Extraction failed: 7')
   expect(comp.modify(strlen, right(lab4))).to.eql('Upcasted 8')
   expect(comp.set(7, left(5))).to.eql('Extraction failed: 5')
   expect(comp.set(7, right(lab4))).to.eql('Upcasted 11')
 })

describe('Either', () => {
  const r5 = right(5)
  const l5 = left(5)
  it('gets mapped', () => {
    expect(r5.map(t => t+1)).to.eql(right(6))
    expect(l5.map(t => t.toString())).to.eql(left(5))
  })
  it('flatmaps', () => {
    expect(r5.flatMap(t => left(t+1))).to.eql(left(6))
    expect(l5.flatMap(t => right(t.toString()))).to.eql(left(5))
  })
  it('cats', () => {
    expect(r5.cata(null, identity)).to.eql(5)
    expect(l5.cata(identity, null)).to.eql(5)
  })
})

 it('gets mapped', () => {
   expect(r5.map(t => t+1)).to.eql(right(6))
   expect(l5.map(t => t.toString())).to.eql(left(5))
 })

 expect(r5.flatMap(t => left(t+1))).to.eql(left(6))

 (ln: Labelled<number>) => ln.x === 0 ? right(ln.label) : left(ln.x),