# File lib/facet/string/first%3D.rb, line 13
  def first=(str, separator=$/)
    separator = /\n+/ if separator == ''
    separator = /\s+/ unless separator
    separator = Regexp.new( Regexp.escape( separator ) ) if String === separator
    i = self.index(separator)
    return nil unless i
    self[0...i] = str
  end