An exception to indicate that the solution of an expression is not yet known
Furthermore, it cannot be determined whether or not the expression is even solvable. When this
exception is thrown, a backfill record is placed on the encoded resolution indicating that
resolver must attempt to solve the expression again, once the encoding is otherwise complete.
This is needed, most notably, when an encoding depends on the address of the next
instruction, because the length of the current instruction is not known until resolution has
Backfill becomes a possibility when an expression depends on a symbol that is not (yet) defined.
Thus, as a matter of good record keeping, the exception takes the name of the missing symbol.