What is to work (over) mentally; to cogitate; to ruminate; -- usually with over; as, to mull over a thought or a problem. [Colloq. U.S.] called?

mull — To work (over) mentally; to cogitate; to ruminate; -- usually with over; as, to mull over a thought or a problem. [Colloq. U.S.]. (Webster's 1913.)

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