a one-sided ergm()-style formula with the terms to be evaluated

must contain one binary ergm term, with the following properties:

• dyadic independence;

• dyadwise contribution of 0 for a 0-valued dyad.

Formally, this means that it is expressable as

sum[i,j] f[i,j] (y[i,j]),

where for all i, j, and y, f_{i,j}(y_{i,j}) for which f[i,j] (0)=0. For convenience, the term in specified can be a part of a simple logical or comparison operation: (e.g., ~!nodematch("A") or ~abs("X")>3), which filters on f[i,j] (y[i,j]) %OP% 0 instead.