Restore chaining?

[lgarron]

e.g.

parse("[F: [R, U]]').invert().expand().structureEquals(parse("F U R U' R' F'"))

Also see #7 (a special case for toString() and #9 (providing convenience for when you just want to manipulate strings.

This is also only easy to provide for built-ins, although Javascript technically supports modifying class prototypes downstream from the original definition.