The `iff_gen()`

sets up the bidirectional if-and-only-if relationship. (Of course there is a unidirectional `if_gen()`

as well.) In the above example, if the point `xy`

is true then any of the ships connected to that point exist. And if none of those ships exist, then the point `xy`

must be false. The `iff_gen()`

function in general takes a pair of sets of terms, and each collection of terms has a "mode" of `any`

or `all`

to determine if just one term must be true or if all of the terms must be true. There was no need to set `modeA`

here because there was only a single term so the any/all distinction is meaningless.