This is an example of a library. You may see a list of other libraries used on Rosetta Code at Category:Solutions by Library.
Calc is a calculator inside Emacs. Calc is part of GNU Emacs 22, or a package for XEmacs. Calc presents itself as an interactive calculator, but also provides a library to Emacs Lisp program.
Calc has more numeric types than plain Emacs Lisp. Calc has big integers, big floating-point numbers, rational numbers, complex numbers, and more. (For contrast, plain Emacs Lisp can only handle integers and floating-point numbers of limited precision.) Emacs Lisp with Calc can solve tasks requiring these types of numbers.