1. A computer-implemented method of guiding a user in designing a product defined by data, the method comprising: using at least one computer, examining items of the data to detect whether each item conforms to rules or violates the rules; using at least one computer, performing at least one action in response to a detection of a rule violation; displaying the data to the user in a manner emphasizing at least one particular data item that caused the rule violation; receiving any new data input from the user; and repeating the examining, action performing, and displaying steps using the new data; wherein the data displaying step includes displaying data defining a current state of the product in a tabular format including cells containing the data, including displaying data detected to have caused the rule violation in a first manner that is visibly distinct from a second manner of displaying other data that is not detected to have caused the rule violation.