Wouldn't one, at some point, also include validation through
the Valid() or RangeLow() method for negative values?


yes, multiple checks are good, but for me best is:
- to prevent invalid input values for reduce user input invalid messages
- put check into the fields for check programmatic errors

A Eiffel approach for any value stored is good,
but best of the best
is define a strong variable data type,
valid only into a specific data values domain.

For example a string domain with a regular expression like data domain,
when the program write a value on a string variable,
it validate the value in the domain automatically.

But i think a like to this language need a 100GHz CPU.

Fabio