text: const sass = require('node-sass')
  
  module.exports = {
    '_log1p($x)': x => {
      if (!(x instanceof sass.types.Number)) {
        throw "$x: Expected a number."
      } else if (x.getUnit()) {
        throw '$x: Expected a unitless number.'
      }
  
      return new sass.types.Number(Math.log1p(x.getValue()))
    }
  }