This introduces a `connected(x)` function, which is simply the same as
`x:x`. It's occasionally useful if `x` is a long expression. It's also
useful as a building block for `root(x)` (coming soon).
It makes sense to omit either of the arguments of the `..` operator,
even though `..x` is equivalent to `:x`. `x..`, with a implied right
argument of `heads()` is more useful.
