Type declarations for records (incuding the private records) can
now be followed by a "witness": a set of values for the record
fields that must satisfy the type invariant (if any). The fields
must be initialized with pure terminating program expressions.
The current syntax, proposed by Martin, is

    type t 'a = { f: ty1; g: ty2 }
      invariant { J[f,g] }
      by { f = e1; g = e2 }

The generated proof obligation is the VC for

    let g = e2 in let f = e1 in assert { J[f,g] }

In absence of an explicit witness, an existential proof obligation
"exists f,g. J[f,g]" is produced.