In Belarusian, a collocation of “па факце” is used in the sense of “according to (actual) fact; actually; in reality”. Sometimes, a wrong “па факту” can be seen instead, due to influence of Russian, where “по факту” is valid.
I have created a simple rule as follows:
<rule default="on" id="pa_fakcie" name="Словазлучэньне: «па факце»">
<pattern>
<token inflected="no">па</token>
<token inflected="no">факту</token>
</pattern>
<message>Тут імаверна патрабуецца месны склон: <suggestion><match no="1" regexp_match="([пП])а" regexp_replace="$1а" /> факце</suggestion></message>
<short>«Па факце», а не «па факту»</short>
<example>Прыпыніць расьсьледаваньне па факце сьмерці пратэстоўца.</example>
<example correction="па факце">А <marker>па факту</marker> атрымалася, што сіл няма.</example>
</rule>
The rule works (tested in the standalone version, snapshot of 26 April), and I suggest including it into the Belarusian rule set.
A full grammar.xml is uploaded here (I can’t see a way to attach a file to the post):