Casting should be implicit in case statements

[brynrhodes]

Ideally, the translator should detect and support discriminated unions:

define function "Implied cast"(vaccination Choice<Immunization,Procedure>):
  case 
    when vaccination is Immunization then vaccination.code
    when vaccination is Procedure then vaccination.code
    else null
  end

In this example, effectively treating the references to vaccination within the then clauses of each case item as though they were of the is type for the item.