-
-/*
- An interaction is a structure containing a "product" of other
- variables. The variables can be either string or numeric.
-
- Interaction is commutative. That means, that from a mathematical point of
- view, the order of the variables is irrelevant. However, for display
- purposes, and for matching with an interaction's value the order is
- pertinent.
-
- Therefore, when using these functions, make sure the orders of variables
- and values match when appropriate.
-*/
-
-
+/* Creates and returns an interaction. If V is nonnull, then the interaction
+ initially contains V, otherwise it is initially empty. */