toUnit(prop, value, unit=px)
  if value is not null
    {prop} unit(value, unit)


abs-pos(top = 0, right = 0, bottom = 0, left = 0, unit=px)
  position absolute
  toUnit 'left', left, unit
  toUnit 'top', top, unit
  toUnit 'right', right, unit
  toUnit 'bottom', bottom, unit