Adding constraints to transforms

[ekshaks]

Format: lhs [constaints] -> rhs
data:

1, ['a', :foo{ :bar, :baz[ 42, :qux[ 99, 'e' ] ] } ] ;
x = (1, ['a',  'foo' : {
                         'bar' : {'baz' : [42, 'qux': [99, 'e']] }
                         }
      ])

(int | 
    (str | (foo & baz & ($num | qux & $qux ) 
            ) 
    )
 )
[type($num) == int]
->
($num | $qux)

The format seems user-friendly;

• need to fix the constraint language
• update the parser (instead, can we add constraints as a lambda?)