1. A knowledge engineering tool comprising a computer having memory for storing a knowledge base, means for interpreting the knowledge base to run an advisory consultation to determine the value of any selected goal expression requested by the user and the value of any subsidiary goal expression implicitly required to determine the value of said selected goal expression, wherein said knowledge base includes facts expressed as expressions equivalenced to corresponding values, rules including premises having logical operations and corresponding conclusions concluding at least one value for a selected expression, meta-facts equivalent in form to facts but prescribing the manner in which facts and rules should be used, and declarations defining whether certain of said expressions are single-valued or multiple-valued, and wherein said means for interpreting said knowledqe base includes means for determining the value of any selected goal expression, means for searching the knowledge base for occurrences of the selected expression, means for invoking and chaining said rules concluding a value for the selected goal expression, means for evaluating said logical operations in the premises of the invoked rules, means for terminating the searching of the knowledge base for a single-valued expression when a substantially certain value is found, and for a multiple-valued expression when all values for the expression are determined, and means for conveying to the user said value of said goal expression.