Tau: the true circle constant

This gem handily defines the true circle constant:

Math::TAU #= 6.283185307179586..

Additionally it expands the BigDecimal/BigMath module:

require 'bigdecimal'
require 'bigdecimal/math'
include BigMath

puts TAU(15)

See http://tauday.com to find out what it's all about.