optional match

[WardCunningham]

We add parse rules to recognize an optional match.

optional match (mngr: Employee {name: "H. G. Cunningham"})

This is presently producing optnl in place of match.

 query
 | optnl
 | | node
 | | | elem
 | | | | bind "mngr"
 | | | | type "Employee"
 | | | | prop "name" H. G. Cunningham
 | | chain
 | end

This is emitting "code" that has lost the fact that we want an optional match.

{
  node: { bind: "mngr", type: "Employee", prop: [ "name", "H. G. Cunningham" ] },
  chain: {}
}

We hesitate here knowing we can emit whatever kind of code we want to apply with the desired semantic. This has been elusive.