>This challenge was inspired by a programming blog I frequent. Please see the original post here: [A Programming Puzzle](http://programmingpraxis.com/2013/06/28/a-programming-puzzle/)

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# Challenge
Define a function \$f:\mathbb{Q}\to\mathbb{Q}\$ such that \$f(f(n)) = -n\$ for all non-zero integers \$n\$, and where \$\mathbb{Q}\$ is the set of rational numbers.

# Details
In whatever language you prefer, please define **one** function or program \$f\$ that accepts as parameter **one** number \$n\$ and returns or outputs **one** number \$f(n)\$. 

Input may be provided through whichever mechanism is most natural for your language: function argument, read from STDIN, command-line argument, stack position, voice input, gang signs, etc. 

Output should be a return value from a function/program or printed to STDOUT.

I would like to restrict answers to functions that do not take advantage of program state or global memory/data that is visible from outside of the function \$f\$. For example, keeping a counter outside of \$f\$ that counts how many times \$f\$ was called and just doing a negation based on this count isn't very challenging or interesting for anyone. The decisions \$f\$ makes should rely only on data within \$f\$'s lexical scope.

However, this restriction is probably inappropriate for some stack-oriented languages or other types of languages that do not distinguish these types of data or scopes. Please use your best judgement to keep with the spirit of this challenge.

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# Scoring
Standard code golf rules apply - your score is the number of **bytes** in your source code.

The minimal answer requires the domain and codomain of \$f\$ to be a subset of the rationals \$\mathbb{Q}\$. If you restrict your domain and codomain of \$f\$ to the integers \$\mathbb{Z}\$, then your score is the ceiling of 90% of the number of **bytes** in your source code.

# Tiebreak
In the event of a tie, the following will be used in order:

1. Fewest number of printable non-whitespace **symbols** in your source code
2. Earliest date and time of answer submission

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You are not required to support arbitrarily sized numbers. Please interpret the sets \$\mathbb{Z}\$ and \$\mathbb{Q}\$ as datatypes in your chosen language (typically integer and floating point, respectively).

If your solution relies entirely on the underlying structure or bit pattern of a data type, please describe its limitations and how it is being used.