11. A computer system comprising: a processor, a computer readable memory, and a computer readable storage medium; first program instructions to establish a co-editing session for a first user and a second user to co-edit a document, wherein the document is a spreadsheet; second program instructions to receive a first modification of the document from the first user; third program instructions to transmit a second modification of the document from the second user to the first user, wherein the second modification eliminates the first modification; fourth program instructions to, in response to a quantity of subsequent modifications, after the first and second modifications, exceeding a predetermined value, initiate an action to prevent further modifications to the document; fifth program instructions to store modification changes to affected cells in the spreadsheet in a cache; sixth program instructions to display 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; seventh program instructions to selectively lock 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 eighth program instructions to, in response to detecting a prohibition against the selective locking of the first subset of cells, issue a message to notify a user of the prohibition, wherein the first subset of cells retain their editability; and wherein the first, second, third, fourth, fifth, sixth, seventh, and eighth program instructions are stored on the computer readable storage medium for execution by the processor via the computer readable memory.
11. A computer system comprising: a processor, a computer readable memory, and a computer readable storage medium; first program instructions to establish a co-editing session for a first user and a second user to co-edit a document, wherein the document is a spreadsheet; second program instructions to receive a first modification of the document from the first user; third program instructions to transmit a second modification of the document from the second user to the first user, wherein the second modification eliminates the first modification; fourth program instructions to, in response to a quantity of subsequent modifications, after the first and second modifications, exceeding a predetermined value, initiate an action to prevent further modifications to the document; fifth program instructions to store modification changes to affected cells in the spreadsheet in a cache; sixth program instructions to display 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; seventh program instructions to selectively lock 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 eighth program instructions to, in response to detecting a prohibition against the selective locking of the first subset of cells, issue a message to notify a user of the prohibition, wherein the first subset of cells retain their editability; and wherein the first, second, third, fourth, fifth, sixth, seventh, and eighth program instructions are stored on the computer readable storage medium for execution by the processor via the computer readable memory. 12. The computer system of claim 11 , further comprising: ninth program instructions to set the predetermined value according to a policy rule for a document type that describes the document, wherein the policy rule defines how many times a particular type of document can be modified; and wherein the ninth program instructions are stored on the computer readable storage medium for execution by the processor via the computer readable memory.