  We describe an efficient scheme for evaluating higher order contributions to
primordial cosmological perturbations using the "in-in" formalism, which is the
basis of modern calculations of non-Gaussian and higher order contributions to
the primordial spectrum. We show that diagrams with two or more vertices
require careful handling. We present an implementation of the operator
formalism in which these diagrams can be evaluated in a simple and transparent
fashion. We illustrate our methodology by evaluating the correction to the
primordial gravitational wave spectrum generated by scalar loops, a 2-vertex,
1-loop interaction. We then look at a generalized $N$-point, 2-vertex diagram.
