1. An expert system shell comprising: (a) a computer having a display device, an entry device, and a memory for string a knowledge base; said knowledge base comprising: (1) variables having values represented by tables of probabitlity distributions keyed by zero, one or more formal parameters; (2) functions defining relationships between the values of each dependent variable and the values of its corresponding argument variables; (3) means for computing the probability distribution of the values of a dependent variable from the probability distribution of the values of the corresponding argument variables; and (b) means for propagating the consequences of a change in the value of a variable to maintain the functional relationships among a selected subset of the dependent variables.