Invoking helper function via dot rather than box

[k-mouline]

The following lines:

fun grandpas [p: Person] : set Person { p.(mother+father).father }

pred NoSelfGrandpa { no p: Person | p in p.grandpas }

test expect { nsg : {NoSelfGrandpa} is theorem }

Produce the following error:

  expected: (or/c node? integer?)
  given: #<procedure:...nchmarks/gp-nsg.frg:34:30>
  in: the 1st argument of
      (->
       (or/c node? integer?)
       #:info
       nodeinfo?
       #:op
       symbol?
       node/expr?)
  contract from: 
      (function intexpr->expr/maybe)
  blaming: <pkgs>/forge/lang/ast.rkt
   (assuming the contract is correct)
  at: <pkgs>/forge/lang/ast.rkt:129:18```

[tnelson]

I don't believe this isn't something we want to support at the moment, although IIRC Alloy permits it. We do need a better error message, and can likely adapt the work we did to add errors for invoking helpers with the wrong number of arguments. Self-assigning.