Complete transfer function for x := new

Forgot to add the part that propagate the results of the operator and
tests to see if something has changed.

See #2 and #9.
Add transfer function for x := y

See #2 and #10.
Add transfer function for x.sel := y

See #2 and #12.
Make the definition of iss more precise

Incorporate the check for compatibility.

See #2, #8 and #12.