1. A method of mediating a co-editing session for a document, the method comprising: establishing, by one or more processors, a co-editing session for a first user and a second user to co-edit a document, wherein the document is a spreadsheet; receiving, by one or more processors, a first modification of the document from the first user; transmitting, by one or more processors, a second modification of the document from the second user to the first user; and in response to a quantity of subsequent modifications, after the first and second modifications, exceeding a predetermined value, initiating, by one or more processors, an action to prevent further modifications to the document; storing, by one or more processors, modification changes to affected cells in the spreadsheet in a cache; displaying, by one or more processors, only the modification changes to the affected cells in the spreadsheet in a first visual display used by the first user and a second visual display used by the second user; selective locking, by one or more processors, a first subset of cells in the spreadsheet while allowing a second subset of cells in the spreadsheet to remain unlocked, wherein locking the first subset of cells prevents the first subset from being further edited by the first and second users, and wherein allowing the second subset of cells to remain unlocked allows the second subset to be further edited by the first and second users; and in response to detecting, by one or more processors, a prohibition against the selective locking of the first subset of cells, issuing a message to notify a user of the prohibition, wherein the first subset of cells retain their editability.