Required interfaces with initialized variables

[pefribeiro]

@madielfilho raised an issue with the use of initialized variables in required interfaces. Calling of an operation that requires such a variable does not lead to it being assigned that value, which is expected, since the operation does not declare, but rather requires the variable.

We should perhaps disallow such use of required variables with initial values via a well-formedness condition? @alvarohm, do you have an opinion regarding this? I don't recall this having been discussed before.