During the past half century, the number and accuracy of experimental techniques that can deliver values of observables for biomolecular systems have been steadily increasing. The conversion of a measured value Qexp of an observable quantity Q into structural information is, however, a task beset with theoretical and practical problems: (i) insufficient or inaccurate values of Qexp, (ii) inaccuracies in the function Q(~r) used to relate the quantity Q to structure ~r, (iii) how to account for...