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In page Mieszko II Lambert:

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Due to the death of Thietmar of Merseburg, the principal chronicler of that period, there is little information about Mieszko II's life from 1018 until 1025, when he finally took over the government of Poland. Only Gallus Anonymus mentions the then Prince on occasion of the description of his father's trip to Rus in 1018: "due to the fact that his son (...) Mieszko wasn't considered yet capable of taking the government by himself, he established a regent among his family during his trip to Rus". This statement was probably the result of the complete ignorance of the chronicler, since 1018 Mieszko II was 28 years old and was already fully able to exercise the power by himself.