class Rational

def to_d(precision)

# => #
r.to_d(3)
# => #
r.to_d
# => (7077085128725065/2251799813685248)
r = (22/7.0).to_r

limit the amount of significant digits.
Converts a Rational to a BigDecimal. Takes an optional parameter +sig+ to

r.to_d(sig) -> bigdecimal
r.to_d -> bigdecimal
call-seq: 
def to_d(precision)
  if precision <= 0
    raise ArgumentError, "negative precision"
  end
  num = self.numerator
  BigDecimal(num).div(self.denominator, precision)
end