# File bigdecimal/lib/bigdecimal/math.rb, line 156
  def log(x, prec)
    raise ArgumentError, "Zero or negative argument for log" if x <= 0 || prec <= 0
    return x if x.infinite? || x.nan?
    one = BigDecimal("1")
    two = BigDecimal("2")
    n  = prec + BigDecimal.double_fig
    x  = (x - one).div(x + one,n)
    x2 = x.mult(x,n)
    y  = x
    d  = y
    i = one
    while d.nonzero? && ((m = n - (y.exponent - d.exponent).abs) > 0)
      m = BigDecimal.double_fig if m < BigDecimal.double_fig
      x  = x2.mult(x,n)
      i += two
      d  = x.div(i,m)
      y += d
    end
    y*two
  end