Fun math puzzle: cin(X)

[Albert Chan <albertmcchan@...>]

Comes across a fun math puzzle. :-)

Design a function cin(X), such that cin(cin(cin(X))) = sin(X)

cin(X) must be smooth function, and at least 10 digits accurate.

In other words, cin(cin(cin(X))) / sin(X) = 1.00000 00000

What is the value of cin(2.019) ?