Sarason's generalized interpolation theorem gives the form of an operator $R$ on a model space that commutes with the compression of the shift operator. This lecture will discuss an analogous result when $1-RR^*$ has a finite number of negative squares. An indefinite version of Nudel'man's problem is first used to show that $R$ satisfies an operator identity involving a finite Blaschke product. An independent study of root subspaces in ${\mathcal H}(B)$ spaces is then used to deduce the form of $R$ from the operator identity.