Does de + um not always become dum?

In the section on periods of time, there is an expression “Ela precisa de um segundo”. Why is the de + um not combined to form dum, as described in the section on prepositions? Are there certain verbs such as precisar after which this rule does not apply?

@furst.steve, contractions of de with indefinite articles are generally seen as optional, so it is idiomatic to use both de um and dum interchangeably in applicable circumstances. This is not observed with the preposition em, that should preferably always be contracted in European Portuguese. Contractions with definite articles are also generally required, regardless of the preposition.

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