Computing the ideal below in a cyclotomic field

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I have a cyclotomic field L, and I define K as a decomposition field for an ideal (non prime) I of OL. ( I made the decomposition field function ). I know that K is a subfield of L, but it just considers it as an extension of Q. I would like to calculate the intersection of I and OK or of I and K, which is supposed to be the ideal below I in K. I've tried the "intersection" function, but it doesn't work because they don't consider OK as an L-ideal. I've also tried the ideal_below function, but it tells me that a NumberFieldFractionalIdeal does not have the method.

Thank you for your help.