lusolve

# File ext/bigdecimal/lib/bigdecimal/ludcmp.rb, line 66
  def lusolve(a,b,ps,zero=0.0)
    prec = BigDecimal.limit(nil)
    n = ps.size
    x = []
    for i in 0...n do
      dot = zero
      psin = ps[i]*n
      for j in 0...i do
        dot = a[psin+j].mult(x[j],prec) + dot
      end
      x <<= b[ps[i]] - dot
    end
    (n-1).downto(0) do |i|
      dot = zero
      psin = ps[i]*n
      for j in (i+1)...n do
        dot = a[psin+j].mult(x[j],prec) + dot
      end
      x[i]  = (x[i]-dot).div(a[psin+i],prec)
    end
    x
  end
end