-convert-to(to-unit, value, context = 16px)
  from-unit = unit(value)
  return value if to-unit is from-unit
  if to-unit in ('em' 'rem')
    return value if from-unit in ('em' 'rem')
    return unit((value / context), to-unit)
  if to-unit is 'px'
    return unit((value * context), 'px')

_strip-units( val )
  unit( val, "" )


rem(value)
  -convert-to('rem', value)

em(value, base = @font-size)
  -convert-to('em', value)

px(value)
  -convert-to('px', value)