To rule across the entire frame (from left indent to right indent if these are specified):

leaf.Rule w    © where <w> is the line weight in points
leaf.Rule 1.5  © a solid black line

A vector of values gives: (line-gap-line-gap .... ) so that:

leaf.Rule 1 3 2

is ‘thin-gap-thick’.

In this release, rules are always solid and black. Possible future enhancements would be colour and line-style.