# Can I amend the preamble from outside the preamble?

I am writing a document (mydoc.tex) that will include a section as a separate document (mysection.tex) using \input{mysection.tex}.

If I want to include something in the preamble (specifically \DeclareMathOperator{}{}), must this be done in the preamble of mydoc.tex or can I include it in mysection.tex?

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