postgresql & Fulltext & ranking & my own functions

Hello,

Documentation says : (http://www.postgresql.org/docs/9.5/static/textsearch-controls.html#TEXTSEARCH-RANKING)
"The built-in ranking functions are only examples. You can write your own ranking functions and/or combine their results with additional factors to fit your specific needs."

The build-in seems not enough for my use case. How to write my own ranking functions ?

I would like to create something like  ts_rank_cd( tsv, 'foo & bar', my_new_normalization_integer)

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pg 9.5