female = null #yes, this is necessary

male = { n |
  true? n == 0
    { 0 }
    { n - female male(n - 1) }
}

female = { n |
  true? n == 0
    { 1 }
    { n - male female(n - 1 ) }
}

p 0.to(20).map! { n | female n }
p 0.to(20).map! { n | male n }